Yeah, 6ohms should be fine.  The power transfer on your rear speakers is not going to be as efficient as the power transfer to the impedence matched speakers, but that's by far the lesser of two evils.  Surrounds tend to not do a whole lot, so you should be in fine shape.
I've got a 4ohm center and the other four are 6ohm but my receiver only has a 6 or 8 ohm setting... hehe.