You should only use a M22 as a center if you can mount it vertically. If you want to use a horizontal center, use a speaker that is designed for that purpose - either the VP100 or VP150.
That should be a wonderful set up for that room! Which sub are you using? Make sure you have a capable one; the M22's deserve it.
Personally, I was not blown-away by the move from 5.1 to 7.1, but I am using QS8's as surrounds. I think you'll be just fine with the M2's. People are going to try to talk you up to the QS series, though.
Your placement in the front is probably going to be driven somewhat by proximity and relationship to your display. Shoot for tweeters around ear level relative to your listening position for L/C/R. For the surrounds, I think that 1-3 feet above ear level is about right.
Hope you will enjoy! I look forward to the pictures!
