Well, if it's a vermin swarm, it would take the Con damage, right? I'm not sure I'd want to make it quite so fiddly.
Methinks the problem might be you're suggestion looked like a single target attack, and those don't work on Swarms (since killing a single bug among thousands makes no real difference).
I was thinking it'd be an area effect attack, probably a burst from the Myconid. Although we could make it a ranged burst. Something like a "puffball" that can be projected 40 ft., then explodes in a 10 ft. burst?
That'd solve the "deadlier against swarms" problems as AoE attacks do double damage against swarms if I remember the rules rightly.
Hmm... maybe we could kill two vermin with one special attack and make the "Venom Quills" an exploding pesticide bomb?
Nah, I like the poison version better, 'though we could make them more effective against Vermin.
How's this:
Spores (Su)
Pesticide: As a standard action, a shroomling can spit out a puff of spores with a range of 40 ft. which explodes in a 10 ft. burst. All creatures within the burst must succeed at a DC X Fortitude save or be nauseated for 1 round (
?) and then sickened for 1d4 rounds (
?). These spores do 3d6 damage (
?) to any Vermin within the burst, or 6d6 to any swarm of Vermin. The damage is halved if the Vermin succeed at the Fortitude save.
Quills
Venom: A venom quill does normal shortspear damage and transmits a nasty poison (injury, DC X Fortitude save, initial and secondary damage 1d3 Con, or 1d6 Con if the target is a Vermin). This venom is supernaturally fast, so the secondary damage takes effect after 1 round, not the 1 minute delay of normal poison.