Take a look at this article, especially the last comment. All identical speakers is the absolute best for a multi-channel system, theater or audio. A sound may move through the room in anyway imaginable, or be fixed at any place in the room, therefore any two or more speakers may be used to create the sound.
Looking at the picture in Matching Front and Surround Speakers, it is obvious that the jet would sound exactly the same, front or rear in the room, with all identical speakers. Using a timbre matched system, like CM7 front and CM1 rear, should be extremely close, but still not exactly the same. One reason some use larger fronts is for also playing 2 channel audio only, but they are not necessary for multi-channel. That said, I would use CM1's front and rear, and look toward upgrading the subwoofer, as a better sub would definitely be beneficial in that size room.