Well,... I have a MC-414C and 4 MC-4C all are the MK 1 and I think they are very nice small speakers. Dialog from the 414C is outstanding, better than 90% of other Centers I have heard.
Yes I think it will sound nice and no I don't think the rears would be overpowering as long as your system is properly set up (an spl meter is very handy).
Think about using the 3 414C's (L,C,R) and pick up 2 MC-4C's for the surrounds? Though the 6C's would be better for stereo. In any case whatever route you go I'd suggest a sub (like an RBH MS-8.1) that will really make these speakers shine.