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

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.

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.