To echo what's already been said, the 5.1 vs 7.1 argument really boils down to the room size & dimensions. In oddly shaped, large, or rooms with weird acoustics, you can see a benefit in those extra rear channels from increased rear fill-in. But 14x16 is a pretty small room, as far as HT goes. That's not a problem - of course - I just don't think you really will need the extra fill-in that you get from the 6+7 speakers. Just calibrate & balance the 5.1 well, and you'll get that all-enveloping, smooth-transitioning sound that you're after.
My den is about 18x30. I've tinkered with setting up rear 6+7.1 channels in the past. It didn't do much for me. There just wasn't enough of a difference to really make me say, "oh wow!". I run 5.1 with the QS8's and am thrilled with the setup. The dimensionality and transparency of the QS speakers are simply stunning. Audio effects move seamlessly behind us and have an absurd amount of spaciousness and depth. So much so, it's sometimes creepy. We've heard people running around in the room upstairs, above the den. We've heard people talking in the yard behind the den. Fellow NFL fans screaming obscenities from the bathroom adjoining the den. Whispers from the other end of the couch. Cannons firing from all around the room. All thanks to those QS8's. Or the house is haunted.
My advice would be to start with a pair of QS8's now and see how that works for you. You can always add more channels later.
And actually, I *think* that I remember reading somewhere that the 6+7 channels in a 6/7.1 setup are supposed to be normal mono-directional speakers (M22's, M2's, M3's, etc) and not the omni-directional kind like the QS's. But maybe I'm mis-remembering that. Might want to check into that before (if/when) you ever do go 7.1
Good luck! And welcome to the Axiom owner nut-house.