[Bruno]

Swarms are dead simple as they have barely any statistics at all: How fast they Move and if they fly, how many HP of damage will disperse them, and how much/what kind of damage they do. That's literally it. Swarms are a wandering Area Attack, not a monster :)

You can note for color how many animals are in one hex of the swarm.

As for "get a random effect" - it's pretty simple. Write up the different effects (ie the different Summonable Allies), find the percentage chance of each one, and weight the costs accordingly.