Polk rm7 satellites

I would suggest just getting another pair of the Pioneer BS-21's. It's true that there isn't much content in the rear channels but having a more capable speaker back there is still preferred over a less capable speaker. In my opinion the Polk is just to small and can only play down to 120hz. I don't really like satellite speakers and that's what the Polk is. It has a 2.5'' woofer and that's not really any better than a flat screen TV speaker. The Pioneer bookshelf speaker has a 4'' woofer and can play down to 65hz according to the specifications I found. A 4'' woofer is still small but if I had the choice I would rather have the larger woofer.

Another thing to consider is calibration crossover settings. Does your AVR have a global crossover for all speakers or can you select different crossovers for each channel? If you have a global crossover for all channels I would not use the Polks because they need to be crossed over at 130hz which is very high. On the other hand the Pioneers could be crossed over at 70hz to 80hz. Having a global crossover for all your current speakers at 70hz to 80hz would be fine. At 130hz not so much. If you do have different crossover selections for each channel then the Polks will be less of a problem. They just don't produce much bass at all.

Well, the RM7 probably sounds better than a flat screen TV speaker overall, but I agree with everything that Secret Squirrel said. In addition, I think that the RM7 may sound different from the BS21, which would affect the imaging and cohesiveness of the surround soundstage. Personally, I prefer to have an adequately timbre-matched system all around (within reason), but even in cases where the surround speakers don't quite match the front speakers, they should at least be a good match for each other, otherwise certain speakers may draw attention to themselves, which is distracting and makes the soundtrack sound artificial, in my opinion.

How do you mount your left & right surrounds? Would you need to mount your back surrounds on a wall?

That sounds like it may work just fine. Especially being about 5' behind you. Just try and leave a little room between the wall and the back of the speaker. Placing the front baffle of the speaker even with the shelf will also have good results. This will cut down on reflections with the shelf.