There's the mob template in the DMG II from 3.5, if I remember right. I think I converted it ages ago for a zombie & skeleton swarm I was using.
Braiiiins.
Swarms are risky things. I recall our party got mashed by a swarm of goblin children and their mothers. That was a ridiculous day and cleave did not work, their morale did not break.
They can definitely make for some intense scenes, and using your example, points out the care you have to take with them.
Only good thing about a zombie swarm is that it can at least take damage...it's the TINY and diminutive swarms that suck completely.
I think I recall an invisible scarab swarm in a minor paizo adventure that was immune to physical damage.
The player sensibly bailed out of that crypt.
