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.