If we are talking home theater here (?), it
IS important that the center channel be tonally matched to your front speaker pair. This generally means using speakers from the same manufacturer and product line. Rear channel ("surround") speakers are less critical but best performance is still obtained when these are voice-matched to the front speakers.
Check Polk Audio's site for discussion.
http://www.polkaudio.com/home/faqad/...e=selectsurrnd
Note that voice matching is especially critical for DVD-A and SACD surround music.
Impedance is a non-issue as long as your amplifiers are compatible.