Mark, I generally suggest that there should be at least 4' of space behind the listening position for a rear sound field to form with back surround speakers in a 7.1 setup. With your relatively close distance the preference should clearly be for wide dispersion QSs, rather than more direct firing speakers which could be considered as an alternative if say 10' was the distance.
The distance that they should be separated varies with the distance back they are, and a rule-of-thumb is to separate them by as much as they're behind you. So, in your case having them separated by 4-5' and about 2' above ear level should give excellent results.
