Given that this isn't really based off any canon, I think we could "invent" our own alien races if we wanted, possibly based-on, but not directly copying any particular Sci-fi race from popular shows or media. That's just my suggestion, and how I've seen alien or fantasy species handled before. Besides, it leaves it open to some creativity and makes sense with the Frontier settlers/explorers/vagabond kind of feel.

I'm personally thinking something like how Pandora was for Borderlands, it had no actual inhabitants, and everyone who came there is from all manner of places off-world, with all manner of pasts. Not that I'm expecting this to be in any way similar, but hopefully you get the idea.