matching center channel to mains/rears

I think your best sound will come from matching brands

FWIW, I had Paradigm Studio 60 v4 fronts and a CC-590 v4 center. I then sold the Studio 60's, kept the CC-590 center, and moved in Dali Helicon 400's MkI for the fronts. Based on all that I had heard and read, I was expecting to be dissapointed with the mismatched fronts/center, but I really wasn't at all. Is it ideal? Probably not but don't automatically assume just because you "upgrade" your fronts you automatically have to plan on a new center channel also. Try it first, and you may be perfectly satisfied with mismatched fronts/center. I would like to eventually get a Helicon 200 center, but I have to say, that the Dali's mate quite will with the Paradigm center.

IMHO, matching the fronts to the center is not nearly as big of an issue as it is constantly made out to be on the forums.

I would definitely stick with ML in order to match the tonality and speed of your Summits... either the Stage or Motif. I have Summits with a Motif center and couldn't be happier.

Concure - MATCH or simply stay 2 channel up front. You wouldn't do a mismatched left and right speaker, so don't mismatch the speakers from left to center to right, either!
Quality over quantity as a default.
Wait till you find or can afford the center, or do a more modest "matched" system for the HT dubties - and leave the 2 left and rights for 2 channel only. Otherwise, stay with the stereo pair.