howabout if, same brand but of diffrent series, say, MA's silver on front and bronze for surround?
speaker manufacturers often have their own "signature" sound that makes them different from other brands. you could probably get away with it even if you use different series. even the use of different size on the same series is possible since most AVRs have individual level adjustments for the LCR and surrounds.
if you have no other choice, timbre matching can still be accomplished even if you use different brands IF your AVR has individual tonal level adjustments and some help with active system adjustments like Yamaha's YPAO and Audyssey EQ systems.
this may prove to be a bit of a hassle for a system owner trying to maximize his setup, that's why the recommendation of sticking to the same brand of speakers in an HT system has been highly recommended.