I want to build a swarm, but based on what I'm seeing in the rules they're really no more than a nuisance for the average adventuring hero. For example, a swarm of rats or bats would be pretty gross in real life, but assuming my understanding of the rules is correct, the attack would mean nothing to someone with even just a couple of points of armor.
How do you make a swarm of small critters actually threatening?
Also, is there a rule of thumb for converting a single critter into a swarm? I don't see a template in the book, and there isn't a single small insect or piranha entry for me to compare them singly vs. swarmly.