Melo,
You'll be OK with the M3ti as a center in that size of room.
If you sit in the sweet spot between the two main surrounds (at the sides), the M3ti's will be fine as surround speakers (and at the back). However, if you or others are sitting in different locations in the room (out of the sweet spot), the QS4s or QS8s will deliver a much greater sense of envelopment in the surround soundfield with movies and with music. I prefer them for both music and soundtrack applications but certainly there isn't absolute agreement on this. Movie soundtracks are mixed with multipolar-type surrounds whereas music-only DVD-A and SACD mixing sessions are done with directional speakers.
Your Yamaha will be fine driving the combination of all seven speakers.
Regards,
