I've investigated closer what you were saying about multiple messages about monster getting hit by clouds. Thing is that when you don't see monster, it is still hurting, in case of noxious cloud that message will be displayed when monter gets into your los, then you see all poison hits at once.
OTOH, it *might* be that clouds damaging monsters works on very similiar rule to regeneration. When you inspect, f.e., regenerating SP closely, you'll notice that every few turns, a few points regenerate, not 1 or more points every turn.
Question about clouds stacking is important cause if you cast 1 ice cloud, then 1 water cloud from adjacent square (the two balls are, say, radius=6

), and if they do not stack upon each other, then there is a huge waste of mana here.