Seems to me that for both of these races, a fundamental assumption is the very nature of the alignments they represent. Applying the K.I.S.S. principle, I think sticking with Planescape definitions is the way to go.
However, to delve into the forum morass that is alignment: It seems there are two types of chaos being discussed. There is the Chaos-as-Entropy, ie chaos is destructive. Pure Chaos is an orb of annihilation. Very serene and tranquil in and of itself, but the chaos left in the wake of its passing is what is meant by chaos. Then there is the Chaos-as-Random.
I think both are interesting, but both lead to very Different results in lore: I think Chaos-as-Entropy/destruction makes for a better "Monster". Chaos-as-Random makes for a better story or "antagonist". In some respects, 4E cosmology went down the Chaos-as-Entropy path and earlier editions were more Chaos-as-random.
The tranquility of a sphere of annihilation is a very Taoist counterpoint to the Chaos it creates. The white dot in the center of the black half (yin) of the Yin Yang. Perfection of self, Chaos in effects. Here Chaos is Destruction. For Slaad, The Slaad Lords are like the Sphere of Annihilation: Agents of Destruction, but themselves are perfect serenity. The Hierachy (colors) lead to more and more self-control. They begin as free agents, completely free from any idea of being "Slaad" and as they change, become more and more, collective. The SLaad Lords, are a Harmonious collective with one single collective will to destroy.
To keep the Taoist theme, Primus could be a being of Chaos in Self, with Law in effects. The entire Modron hierarchy is a reaction to Primus-as-Chaos-as-Randomness. Primus IS the penultimate Rogue Modron. In this case, Chaos isn't destructive, but rather Random. Rogue Modrons are "special agents" of Primus. Perhaps the randomness is simply that Primus has emotion, humanizing him. Or perhaps he is truly random. Modron hierarchy goes from the perfect drone and as it progresses, they become more personable, more individual, more Primus-like.
Then again, dichotomies within dichotomies makes my brain hurt. Yeah. Planescape. K.I.S.S.