I would leave the LFE set to sub only to minimize any phasing issues between your fronts and the sub, in whatever frequency range they may overlap. I had mine set to both for a while, but after I switched it to 'sub only' the first thing I noticed was how much clearer and controlled the bass output became.
I agree with MDS too - you can leave the receiver set to 8 ohms as the rears aren't going to be driven that hard anyway, but keep a check on it. If there is any problem the reciever should go into protect mode before damage occurs, but I would think this is even unlikely in your case.
Ideally - match all speakers for the same nominal impedance rating - as you may or may not know, these ratings are nominal as a speaker is an inductive load, rather than a resistive one. Like any inductor or coil (that opposes a change in current - in this case, the speaker level signal going to your drivers), it's impedance will vary at different frequencies. If all of your speakers are 8 ohm for example, then it becomes a non-issue, and one less thing you have to worry about.