I have just gone throught the whole process of purchasing new speakers, and also wanted to know about mixing speaker brands. (See thread: centre speaker sounds muffled).
The advice I got was in an ideal world you having all matching speakers is probalby the best. Having said that he went on to say, if you cant match speakers then try to match technology, i.e if you have horn type front and center speakers, try to have horn type speakers in the rear.
Having said all that, although i didnt ignore this advice, rather I couldnt afford to dish out more dosh for new rear speakers; I moved my older front canton cone style speakers to the rear and bought new reference series Klipsch for the front 3. Please read my last post on my "muffled" thread about my results.
hope this helps.