One-off atrocities would be interesting, but they would only be used as a last resort. Even when driven back into the Arc the Inchoroi didn't/couldn't release such creatures, and it is hard to think of a more desperate situation.
It might be that they have plenty of stock laying around and are just unwilling to use them. After all, they conquered hundreds of worlds and would surely have kept left-overs. Just remember that the created weapon-races that we see where specifically designed for Earwa in order to preserve the world and destroy only the humans/nonmen (sranc only eat insects and leave game alone, I don't know is skin-spies and Wracu eat at all). They might be worried that the world they so desperately want to claim for themselves would be decimated by releasing other species into it.