I personally use identical bookshelf speakers all around (including front center), as I use a subwoofers for the deep bass anyway. I recommend that most others do the same. That way, for the same price as towers, one can go with speakers from a higher line. And they also will all be perfectly voice matched that way.
Now, most people mix and match bookshelf and tower speakers, along with a horizontal center speaker, so most people will not have any problems with you doing as you are thinking of doing. In fact, since we are discussing surround speakers rather than the front three, it matters less there than the fact that you are planning on using a different speaker for the center than for the right and left front channels. If it does not bother you to use a different center channel speaker on the front, it ought not bother you to use different speakers for surround use.
As for 5.2 versus 7.2, how much that matters is something upon which there is little agreement. I went from a 6.1 system to a 5.2 system now, as extra rear speakers matter little to me. But this is partly a matter of placement (and as you know from your case, not everyone has the same options due to space), as well as personal preference, and so you must decide for yourself on this point as well. Of course, for the same money, one can get better speakers for a 5.2 system than for a 7.2 system (because with the same price, there is more money per speaker that way), so that may enter into the issue as well, unless you have unlimited funds.
