So, ants with toxic flesh? If they exist in real life (which they probably do, knowing how ants work), then they might add them.
One thing you might want to do is make it so that, instead of being "uneatable" (which would be too overpowered), the 'poison ants' deal DoT ("Damage over Time") to predators that eat them. That way, there will still be a few casualties, just not many.
The problem is that poison as a defense for ants is a pseudo-new concept that might shift the balance too much, and remember, every ant featured in the game is a real-life species of ant (except maybe Gene Thief - I've never actually heard of that one), so if there are no toxic-fleshed ants in real life, then there won't be any in-game, either. But whatever, it's still a cool idea. I hope it gets implemented.
Edit: Wrong form, I think. I thought this way EotU, not Ecosystem.