Andrew, in a 7.1 setup the side surrounds ideally would be directly to the side of the listeners, but especially with QSs, it isn't critical and maybe splitting the distance between the rows of seats would be fine. As to the back surrounds(on the back wall), the distance behind the listeners(as long as it's at least a few feet)also isn't critical and calibration of sound levels and delay will ensure a proper effect at all reasonable distances. The back surrounds should be separated widely enough so that both a left rear and right rear effect can be experienced and in your case 12' might be reasonable.
