We tested the idea when we were researching bit mining deciding on whether or not to invest into the idea.  What would make a mess for graphic rendering works great for bit mining.  We tested nvidia and ati cards mixed in one box.  Another would have 3 different generations of AMD cards in it.  To date, we have found only one card that does not work at all with cgminer (or any other opencl base miner for that matter).

Now for the sake of simplicity, predictable wattage consumption, and overclocking with utilities like MSI afterburner, I'd stick with one card type per mobo.