Ofcourse you can, but matching the three front speakers and amps is best, the surrounds dont matter as much UNLESS you are a multi channel music freak.
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chadnliz has the best point. The front 3 are most important, but if you can match your front to back do it.
Some amps sounds similar and mixing them would not make much difference, unless you go with a bright amp like theta and put it with a sunfire would not be something I'd recommend.
3 weeks ago I did a major cable change replacing all 7 channels which has 9 speakers to it changing the speaker cable. I always had good speaker cable in the front and cheaper in the surrounds. Was this ever a nice upgrade. I was hearing things like I never heard it before. Meaning, the sounds sweeping from the front to back had such a nice blend and flow. The whole system sounded to be harmonizing now with blending timbre. My surrounds are of the same speaker family.
I found a way to diy speaker cable cheap that rivals $1000 to $2000 cables, and saw it as a way to finally bring the whole system to a reference level all the way around.