The center channel is a separate amp. You are not mixing the impedance with the main speakers. You won't have any problem.
Mixing 4 and 8 ohm speakers
I drive a pair of 4 ohm KEF Reference 107/2s with a Proceed HPA-3 amp (500 watts/channel into 4 ohms, 250 into 8 ohms). I'll be adding an 8 ohm KEF R 600C center speaker. The HPA-3 is essentially 3 amps, each with its own power supply, that share a chassis. Need I have concern about mixing output impedances? Of course, I'll match levels with an SPL meter.
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