Your comment requesting us Audiogon users to "be gentle" is not so far from the mark I suppose! The audiophile breed is amoung the most critical on the planet it appears! (no toys to play with when we were kids...)
Anyway, my suggestions would be to not use any center or rears at all if you can't use the same speakers as you have now for those purposes! If you use mismatched speakers, no matter how close, it will only bring the integrity, believability, and coherence of the system DOWN! You'd be best served by simply using your main left and rights with a sub through your pre/pro!(which you MUST use for proper movie playback of DD/DTS).
Another solution is to buy a whole nother set of even less expensive, but more movie friendly/effective HT speakers/satalites. This way you don't sacrifice the integrity of either your HT or music. You could mount 5 or more matching HT speakers around the room(maybe a dedicated matching center over or under the TV/monitor/screen), and potentially have a much more effective HT system. There are compromises here however in terms of set up for best sound, and an EQ might be in the works. Still, the integrity of movie soundtracks isn't what a good 2 channel recording is ultimately. However, If you're happy with what you get from your 2 channel system for movies, you might just consider keeping those two set up "as-is", and integrating the subwoofer. There are of course lots of options. But if you plan on keeping the 2 you have, and doing 5.1 or better, you will need matching speakers, not similar sounding speakers! If you don't want another speakers system for movies, and 2 channel is your priority, you might just keep things as they are. I pressently have a 2 channel set up with sub and processor for my movies, and it sounds great.(but then it's set up well.)
My thinking is that I'd take a quality 2 channel set up(integrated with proper digital pre/pro and such) over not so well executed multi channel every time! So it must balance.