If you evolve into a hostile alien race, with a need for slaves, I think it would be a cool feauture to enslave another species to serve you. You could have them build things for you, live in districts, you can kill them too.
Another cool idea I had was to force a species into bio-mechanical slavery, by manufacturing them into semi-robotic slaves for your own military. You might be able to this with animals, by using them as a cybernetic war machine in your divisions. You could sort of build a ‘union’ this way, combining different cultures together into one whole one.
Yeah, this is an excellent idea. But war isn’t the only option to unify nations or species. Diplomacy is another way. But yeah, your idea is realistic. That’s good. (no, I’m not ironic)