1. They hate bright lights
2. Don't get them wet
3. Never, ever feed them after midnight
The first one makes sense, I suppose, but the other two always bothered me. How would water facilitated muticellular fission work? And where is the extra mass coming from? (This bothered me about the Hulk, too.) And any time is after some midnight. How long do you have to wait? They never make it clear.
I was pondering this again last night when it suddenly occured to me that maybe there is no answer, that the Mogwai are not well defined-- it's an underconstrained problem I'm trying to solve. And that realization brought me a sense of peace about the problem that I had never had before when considering it.