The QS4 are essentially the QS8 with a smaller driver, different crossover point and lower power handling. The VP100 should be adequate as well since your room which isn't that large. I wouldn't see any problems whatsoever in mixing any of Axioms bookshelf, surrounds, towers and center models with each other as long as you stay within reasonble limitations based on the room.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne