You would be feeding them sub bass, and its never a good idea to mix speakers like that on the same channel.. the freq would be off and it would be a wiring nightmare.

Take the 4 channel, and hook the fornt outputs to your components, and the rear ouputs to the 6/5's or 8's. Or bridge teh rear outputs tot eh 6.5's or8's. You wont have l/r if you bridge..but im not sure of the effect that will hav eon imaging