Is there anything wrong with mixing incompatible paints?
So, from my experience...


Wrong? No. Basically, looking at the major art paint options I can't see any that, when combined, would result in a dangerous mix. Some, however, will mix a bit better together than others.
Yes and no. The key to the cracking effect is difference in drying times, some paints dry faster than others and when you apply a fast drying paint over a slower one, the result can be that the upper layer will crack. The "no" part of this comes from the ability of some mediums to adhere better or worse to each other. For example, acrylics use water and oils, well, oil. When you try to apply acrylics over oils, it will crack and split (your desired effect), but it will also tend to flake, which is less desirable. In any event, some mixes will probably work better than others, but there's likely to be some trial and error involved.


So, my suggestion is that if you're going to give a go, do so on some items you don't care about, but that also has the same surface material and try a few combinations using some student-grade paints (much cheaper).