Hmm - that does seem to match their anal-retentive personalities
I would add that they probably are renumbered at very frequent intervals (due to modron death, or just because), where the modrons have to all receive new numbers at some complicated meeting/ritual.
I imagine the modrons as pedantic, brainwashed sods who continually look at what the other modron is doing, achieving happiness by imitating their peers and following their leaders; in addition, there are many meetings and rituals and such that they simply have to do - just because (they have some reasoning for everything, but the logic is really logic for logic). Ironically, you cannot have pure law - someone is (at least to some extent arbitrarily) creating the law in the first place (i.e., Primus).
I imagine the slaadi as incoherent, schizophrenic freaks who act based on momentary emotions, not concerned about the effects of what they did or what they will do (but doing whatever they feel like makes them happy). Ironically, you cannot have pure chaos - there is a hierarchy to an extent (although I don't think slaadi would follow orders very well) culminating with Ssendam and Ygorl.
I also imagine that the modrons and slaadi absolutely hate and despise each other, seeing the other as the lowliest excrement of the multiverse.