Tater has answered this perfectly!
You can be as creative as you like with the swarms, but the most flexible approach is to make up the swarms on small bases and just put them on medium bases when they grow.
I haven't done rules for large swarms just yet... medium swarms are scary enough! (Although I learned that they are real liability when playing against Paul's massed slingers...)
what's this talk about large swarms?
What I did with mine was carefully remove one frenu from the swarm so my two small swarms have three frenu on. The medium swarm is mounted on a 40mm base and has the two extra frenu added so has six and looks a bit more "swarmy"