The eidolon chart is perfect for Zerg. They can adapt by moving points around. The gain something and they lose something. You can make basically all the Zerg with the eidolon chart.
I am on my phone so I will keep this brief... But I think anything with a have mind would essentially have it's connection severed if you attempted to control it. You would have to try to control the Overlord itself in order to control the Zerg because as soon as the connection was severed... They would revert back to the Primal stage. Could make it interesting though, it could potentially blow up in The party's collective face.
Exactly what Tiny Coffee Golem said above. Use wolves (and wolf-pack hunting tactics) with eidolon evolutions, describe them as zerg, and voila, Zerglings.
When you have players using fire to kill them over and over and they begin to mutate for resistance to fire, make their AC drop a little. When players stop using fire so much, make them evolve DR, but lower their HP a bit. If the players adapt to that by committing more resources to single, high-damage hits, then increase their HP but lose the fire resistance.
And only tell your players that what their doing is effective or not. Slowly. You shouldn't have to change mutations too frequently, so you can plan a few of them ahead of time.
If you want to see another "Horde of Alien Locusts thrown into a Fantasy Setting" plot, you might want to look up the Codex Alera and their variation on the trope, the Vord Swarm.
They are just Zerg. Hell, they even have the creep.