I went 6.1 due to an unconventional corner layout and the rear center helped fill in the hole I had between the left and right surround.
The Axiom ceiling mounts will position the speaker the required amount.
I am thinking, looking at your pic, that mounting the surrounds to the side wall and then to the angled wall would also work very well and save a few bucks on the mounts. The QS's come with flush mounts so maybe try them out that way and if you don't like the sound then get the ceiling mounts.
