The Pioneer tower speakers would be good, but, yes, their center looks weak. It could probably sound nice though. If you can somehow accommodate another sp52 tower as a center, that would actually be the ideal.
As for the 15" woofers on the towers, they may not be useless for sheer bass output, but they probably won't sound very good, and they certainly won't dig very deep. I would be scared to get towers with 15" bass drivers for so cheap. The cabinet is going to be hollow, the drivers will be pretty bad, and I don't even want to think about the crossover. CW's are known for being pretty rough speakers, and these are off-brand CW knock-offs! Who knows, they might sound fine, but I wouldn't chance them, not even for surround speakers.
If you need to use towers as the surround speakers, I would just get more of the Pioneer SP52 speakers as you can afford them. Stick with 5.1 for now if you can't afford 7.1, and add two more later when you can. I would try to work the PA-150 sub in there, that sub is a proven performer and a real output monster.