Mark, the space behind your listening position should be enough to make back surrounds worthwhile(note that a mode such as DPLIIx extracts useful back surround content from both 2-channel and 5-channel source material). If you go with 7.1, the side surrounds can best be placed directly to the side of the listening position, since a position slightly farther back isn't necessary to help add a "phantom" back effect, with actual back surrounds being there. The back surrounds should be separated about the same as your distance from them, say about 6'.
As to which speakers to use for back surrounds, since they'd still be relatively close to the listening position, it might be better to use wider dispersion QS8s(or even QS4s)rather than direct radiating speakers.
