Yes. Its easy to fit pretty much anything in the rear though. There is a removable panel that the 6.5" speakers mount on and it can be easily remade out of 1/4" plywood and you could mount any speaker in it. I put a pair of 6x9s in mine last night.
yep, 4's in the front,and in a std cab, 4's in the cab corners by the seatbelts. Ext cabs I think are 6.5 in the back. But as Zac said,easy to cut and make different sized speaker mounts. But 6.5s are great anyways.