We can probably reason that the swarm damage exposes the victim to a larger dose of both acid and poison, perhaps even from deliquescing individuals in the swarm.
I'd prefer just acid plus poison for swarm damage. It just makes more sense to me.
3d6 acid, pison damage of 1d8 or 1d10 followed by 1d4 Dex is ok with me. Sickened for 2d6 minutes also works.
Actually, this raises a good point: we need to specify the size of the poison puddle within Deliquescence. I'd go with 5 ft radius (ie, essentially one 5 ft cube in game terms) for a single gossamer. I might stick with that for every point where a swarm runs up against an object of, say, Medium or larger and, as Arnwyn suggests, a larger volume when the whole swarm dies.