If you're staying with 5.1 for the moment, and doing 80% movies, I cannot recommend the QS series surrounds highly enough. As good as any of the monopoles might be as surrounds, I cannot imagine watching a movie without my QS8s. They are the most unobtrusive, yet immersing surrounds I can imagine. Here is a portion of a post I made over at the HomeTheaterSpot regarding the same issue.
"I cannot recommend the Q'S series speakers too highly. I'm in a room 11.5x18x8 and I'm running M60s/VP150/QS8s. All of them are excellent speakers, but, of the three, the QS8s are best at doing what they are designed to do. They are amazing, enveloping surround speakers. I wouldn't miss the HT experience they provide for anything. My first SCAD/DVD-A player is soon to arrive, and I have no trepidation whatsoever about using the QS8s for surrounds in either format. Though, in the future, I hope to add a pair of M2s, for my 7.1 setup."
Depending on the size of your room, I'd recommend getting the M60s for mains, and the QS series (8s or 4s) as your surrounds. I don't think you'll be sorry.
If you want a monopole for your center then go with an M22 or M2. Since you already have M2s, you could try one in the center position to see if it would do the job, and if you like a monopole as a center speaker.
Keep your M3s for a bedroom stereo set (or sell them if you wish), hang on the M2s to use as your rear surrounds when you go 7.1. As Alan pointed out, the M2s are a good tonal match with the M60s. If you find the M2s lacking in the rear surround role, you can then upgrade if you so desire. But you should give them a try as rears before you do.
"People generally quarrel because they cannot argue." - G. K. Chesterton