I only have a few minutes before I need to head out for something, but I do have some thoughts on this. M2s would seem to be a better match for you, if you want Dolby Atmos imaging to be correct, you will have to replace the QS8s. I know that Dolby said that di/bi pole (and thus quad poles like the QS speakers) would be "ok," but if you could see the facial expression on the guy from Dolby when he said that, you could tell that is was almost painful for him to even say that people could use something that wasn't direct radiating (monopole).
If I was experimenting, I would say to put the QS8s in the rear, and the M2s on the sides and here is why...
With monopole fronts, sides, and overhead speakers, you should be able to get some pretty good object based sound effects in front of your ears, and the "less accurate" effect from the QS8s would be behind you where it wouldn't be as critical since sound behind our ears isn't as discernible as to its exact location.
Height is probably OK at 6', but in the demos I was in, they were, on average, just about ear level on the wall, which would be closer to 4' - 4.5'. This gives a little bit more room between the speakers on the ground/stands/walls and the ones on the ceiling which in turns allows for a bigger "dome" of sound as Dolby put it.
