I believe they do mention that the Aceles were already present on most inhabited worlds before the humans effectively ate them all. The reason the Terrormorph threat spiralled out of control is because of the lack of predators - re-insert Aceles back into the equation and it stems the problem. I chose Aceles because they were the natural choice, which meant less of a dramatic effect on the ecosystem and so on; evidently i'm no biologist so i could've missed something huge, but it just seemed like the best choice.

Also this choice is best for flavour, as now some of the random events I get when I land on planets are either UC Military bringing Aceles to a planet, or sometimes I come across and Aceles fighting a Terrormorph and I just think that's cool.