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You can mix and match. Most AVRs let you set the crossover frequency for front and rear speakers separately, assuming you’re using the sub/lfe out. Just watch out for the usual speaker stuff like attaching high-impedance speakers to an amp that can’t handle it.

There’s some leeway on the rear speakers since they’re mostly used for environmental effects you can get away with a cheaper set. I’d be less inclined to skimp on the center speaker though. You generally want it to be a close match to the front L/R. Though a 4.1 setup could work if your AVR knows how to do it properly.