Craig, that setup sounds(or would sound)fine; one suggestion that I frequently make is to consider the use of a small vertical center, such as the M2, to get wider and smoother horizontal dispersion than a horizontal MTM design. As to back surrounds, again some prefer direct radiators there, while others want still more dispersion and use QSs all around. The 659 or similar should have plenty of power capacity unless you play material with a very wide dynamic range extremely loudly.
On the positioning of the surrounds, those heights wouldn't be a problem and several members here use ceiling mounts. The side surrounds should be directly to the side of the listening position, if possible, and separated widely, almost at the side walls. The back surrounds should be separated about the same distance as they're behind the listening position.
