It won't sound funny but there may be a tonal difference between the L/R and the other channels. That is the price you pay for keeping your 2 channel preamp/amp. The only solution is to have the same electronics for every channel.
It's perfectly fine. You may want to adjust the levels of the speakers if the wattage being fed to the different speakers is materially different (which you should easily be able to do through the AVR). But, in reality, you'll very likely want to do that anyway. For years, in exactly that setup, my surrounds were seeing 105wpc from the AVR, the center 800 from a bridged monoblock, and the mains each 250. The speakers are all different anyway, so it's not like I could make them sound identical even if I cared to try. So I don't. Not a problem at all.