I had this notion for a swarm-based house rule. Swarms made of Diminutive or smaller creatures take double damage from area effects, rather than just 50% extra damage. I was thinking this would balance them against Tiny-sized swarms, since the latter are not immune to weapon damage, whereas Diminutive and smaller swarms are. Also, greater surface area = more damage from area effects?
Makes sense. And makes martials feel slightly less useless using mere splash weapons. That said, not to invoke any sort of "disparity", but it does reward casters more. In essence, the problem you're trying to solve (immunity to weapon damage) can't really be solved by a modification that doesn't especially help those who use weapons. :P
I'm not trying to either help or hinder martials or casters, specifically, although I will admit it does irk me a little that this will benefit casters more. The point of this was to not just allow swarms composed of itty-bitty critters to be objectively better than chunkier swarms. Under this system, the two "kinds" of swarms, Tiny and Diminutive/Fine, at least have different strengths and weaknesses.
Basically, in my home group martials don't have a problem gaining quick and cheap access to powerful nonmagical area weapons (crazy alchemical flamethrowers, to just name one), so that's not a problem. Even if it was though, this is mostly me wanting some kinda intra-swarm balance.
But thanks for the input!