Can I mix ohms in a surround system?

Rev, you certainly can. My rears are 8ohm in-ceiling speakers and my fronts are 6 ohm. So specifically, you can have different impedance speakers on the same amp.

In my opinion, more of an issue is the timbre match of the speakers as opposed to the Ohms. If the speakers are from the same manufacturer, I wouldn't think twice about it.

Cool. What about setup or configuring - do I have to switch anything or it just doesn't care?

The emotiva have separate amp modules per channel thier new ones but you have the xpr series some of thier first amps.i have the xpr 1 monoblocks.wish i would have bought more of them read the reviews  I just rec a pped it and had to call the company ask what the bias was excellent tech support call them and ask.enjoy the music