Is Full-size speakers for L-R Rears advisable?

1 No FS for the rear will not replace a subwoofer.

2 The majority of information comes from the LRC channels. So the need for FS rear speakers is not as necessary. That is not to say that a FS won’t hurt. Remember the old adage nothing replaces displacement.

3 use your best speakers for your fronts.
4 probably not.
5 dealers choice. Depends on the situation.

6 no.
7 no.

1&2 - A subwoofer is especially useful between 16Hz and 40 Hz or so.  The full answer depends on the in-room response of the speakers.  HOWEVER - If you don't use full sized speakers for the surrounds their bass will have to go somewhere.  Having full sized surrounds can reduce the load on the sub and L/R depending on how they are set up.   There's also something to be said for having multiple bass sources being better for overall bass reproduction.

3 -  This is really up to your hearing.  Are the more expensive speakers actually wroth the ask to your ears?

Generally speaking, I've mixed LCR speakers with different drivers but they were all very smooth performers.  If you end up with the same tweeter and similar midwoofer from the same maker you should be OK.  The room correction software should be able to help match below there.

4 - No

5 - You could get a multichannel amp.  Bryston for instance makes a configurable HT amp you can put 3 channels or more into, Parasound makes at least one 3-channel amplifier.  I'm sure others do as well.

6 -  Not at all, but they should all have meters.... <grin>  Just kidding.  You can use your current amp meters as a guide of how much you'll actually use.  Sadly, the surround speakers are more or less effect speakers and rarely have continuous usage.   Many HT receivers have surround amps that are significantly less powerful than the fronts, so this can be a guide.

i have 3 sets of full size speakers for my HT and love it.

You can go nuts with this, but if your priority is 2-channel and you just want a very good HT setup integrated into it without blowing big $$$ this is what I’d recommend. You can just use monitors for the rear speakers as the surround signal carries mostly ambient info so extended bass really isn’t necessary, and I’m also of the opinion that you needn’t spend $$$ on them as again they just mainly reproduce ambient sounds. Something like a $479 pair of Wharfedale Diamond 12.1 monitors are plenty sufficient and you’d be better off putting remaining funds toward other things. Speaking of, you absolutely need a sub for truly involving HT, and I’d recommend something like a Rhythmik F12 that is an excellent performer and will get you down to like 14Hz, and as they sell direct it’s a great value as well.  If you want to spend less something like an SVS SB2000 or SB3000 would also be fine for HT.  Unless you really wanna shoot the lights out on HT I’d just get a good AVR and use that for HT processing and for powering the center/surround speakers. I really like the Yamaha Aventage AVRs for sound quality and reliability, but if you want to up it a level and spend more you could go with Anthem. Integrating this seamlessly into your 2-channel system (switching between 2-channel and HT by pushing one button with the AVR completely out of the signal path for 2-channel) is very easy, but if you’re unfamiliar with how to do it just ask and I’ll go into detail. Incidentally, what stereo preamp are you using now? If it has a HT bypass it makes it easier but still very simple using any line level input if it doesn’t. Don’t sweat power/impedance differences as you’ll correct for that by setting individual speaker volume levels with the AVR. Hope this helps.

A lot of folks don't realize how much bass info is available from the rear channels, at least on a well-engineered surround soundtrack.  I've got a horn-loaded JBL stage monitor with two 15" woofers (combined L+R rear channels via mixer), and get as much bass from good recordings as from L-R front channels.

The original surround sound standard does not include a need for FS speakers for anything other than LR.  All other speakers were 200hz and up. This has been carried through for decades now in the cinemas. Bass has always been handled by sub-woofers. 

The only thing I can add here to the otherwise excellent responses is that rears should be used mainly for ambience and are not meant to carry the load.  Monitor size speakers should be adequate for this.  TBH I have never heard a system that uses floorstanders as rears, so I can’t really justify my claims with experience 

I have full size speakers for front and rear.wih dsp my denon avr 110 has the microphone that evens it all out.atmos is great.some music recording now are in atmos.they have self prowered subs.with class d 1500 watts each.class d is where it's at now for base speakers.the home theater stuff now is better than the theater.16 Chanel sound who needs stero.that should get it going.enjoy the search.with the dsp it's able to control time delay gain ie volumn  lessing the effects of different amplifiers.alot of these processors for home theater have xlr out the back so you can use internal amp or shut that channel off and use external amp.this makes it better for the front and center channel that require more wattage than the rest of the surrounds.you can even set it for stero mode only

I have full range spears at each of the corners (FL/FR/RL/RR) which was a decision made to support the multichannel SACD format which does spec full range rears.

With that said, I have never once wished that I had smaller speakers in the rears.

The original surround sound standard does not include a need for FS speakers for anything other than LR.  All other speakers were 200hz and up. This has been carried through for decades now in the cinemas. Bass has always been handled by sub-woofers. 

@pcolvin - In the mid 1980s I worked in theaters as well as a major manufacturer of electronic equipment and racks for theaters.  I am going to call you out and say state your sources.

I’ve been behind at least a dozen different theater screens and can categorically say that the main LCR speakers were always full range and identical.  Surround speakers were usually different in large part because they had to be mounted up about 12’ high and were quite visible.  Surround speakers were therefore size limited and there were limitations in the high frequency content in the original Dolby Surround tracks, but we can think of them as full range. 

In addition to being behind the screen I also helped calibrate a variety of commercial movie sound processors, my experience ends before Dolby Digital however.  None of them had built in bass management to enable a subwoofer.

The subwoofer you say was always there was most certainly not.  It wasn’t until the limited use of Sensurround in around 1971 that there was EVER a subwoofer in a theater.   And while that was an important achievement it wasn’t until Dolby Digital that a separate low frequency effects (LFE) or bass management was even a normal thing in theaters.  As far as I know the standard is still for full range speakers behind the screen AND subwoofers.  The use of small main speakers is strictly a home thing.

The other weird thing you state is that they were limited at 200 Hz.  No idea where this comes from.  Even when THX started branding satellite speakers as THX certified the THX spec was you used a sealed speaker that went down to 80 Hz. Any higher than that and the bass would be directional.  The sealed requirement was to ensure proper phase and amplitude matching with the subwoofer.

@erik_squires I was the engineering manager for Litton-Westrex until 1983 (when it was sold to Quad-8) which did sound-on-film since the Jazz Singer in 1929, 6-channel film recorders, and record cutting lathes/cutting heads. 

At the time I worked there I was involved with the early days of Dolby and DTS for the theatre, and my engineering staff included Keith O. Johsnon (who created the dbx Sub-harmonic synthesizer which was used to drive subs for the Pacific Cinema Dome in Hollywood) and Terry Beard (of DTS). 

In partial answer, at the time Altec-Lansing A-1’s with the multi-cellular horn was used behind the screens (depending on the size of the theatre you had LR or LCR) and was run from a 35W/chl tube amp, and for Dolby Surround Electro-voice was used (various models) for the surround channels with Ray providing the upgrade to the amplification chain.  In the theatre world if you want to put out something new you have to pay for it yourself: Universal Sensaround was similar in case as Universal paid for the equipment and installation to have it put into the various theaters and removed it afterward.  Subs were not added until the time of Dolby and THX.

Sounds like you worked around the Hollywood area.  Outside of Hollywood, most of the theaters were 35mm optical with A-1s behind the screen and, again, some 35w tube amps. No equalization was done beyond the original Academy curve, which was provided in the amps:

Key Characteristics of the Academy Curve:

• Flat response: The curve is flat from 100 Hz to 1.6 kHz. 
• Reduced bass and treble: The response is attenuated (reduced) at lower frequencies (40 Hz, 7 dB) and higher frequencies (5 kHz and 8 kHz, 10 dB and 18 dB respectively). 

Hence, no need for subs.

Additionally, remember that at that time you were, and still may be, shooting sound through a fabric screen with a bunch of tiny holes in it.  The Academy curve took this into account as it also took into account that you were laying sound tracks onto the film between the sprocket holes and the film frames. 

Back in the 80's Ray Dolby, Tomlinson Holman, and Terry Beard had to prove to the Academy and the theatre owners that changing sound quality would result in increased sales. It took a lot of time, experimentation, and show & tell before what we have today caught on. Terry's use of separate time-synced CDs/DVDs under the DTS (Nuoptix) name brought digital audio into the mix, whereas before Ray had to deal with a encoded stereo and 4 channel optical mixes (unless it was a 70mm release).