I would keep the sides and rears the same. In that size of room I'm not sure how much benefit the rears will be though. My room is the same width and about 3 ft longer and I'm utilizing a 5.1 setup. If I had closer to 2500+ cu ft, I'd consider rears but really they need to be placed 3-4 ft behind your seating position to work best.

Why not go 5.1, try it out first, then if you feel you still want to try a 7.1 system, go ahead and order a couple more "Q's" afterwards?

Half of communication is listening. You can't listen with your mouth.