Depends on the radiator how much it'll drop temps/limit flow. A single probably won't decrease the temps that much, but it won't hurt, that's for sure. Your second question. From personal experience and reading around here on the forums (and elsewhere) I haven't found anything that says that push or pull is any better. From what I've read, it doesn't make a bit of difference and even putting fans on both sides (push/pull) makes little difference than to have it one way or the other.