Sounds like minions might be a good option for you. Quarter the health, triple the number, and that’s basically how minions function according to Chapter Running The Game. Plus, “swarm” normally makes people think of “weak alone but strong together”, which I feel minionizing monsters can help do. This splits your one swarm monster into 3, helping the feel that this swarm is, yes, collective, but still made of a mass of individuals, and allows the swarm to split rather that always be one unit So, instead of 1 with 18 HP, 3 with 4 or 5 HP depending on how you round. Instead of 2, 6. Instead of 3, 9. It builds up fast. As for the stats themselves, these seem pretty good for your swarm of centipedes. As for sickened, if you really want a mechanical reason, you could use Attribute Substitution Might Into Entropy, but I’d not stress yourself too much regarding that.