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.