A while ago I discussed the way that Star Wars, being inspired by movie serials like Flash Gordon, takes place on a very small scale: even though there’s a whole galaxy at stake, the Empire is basically one fleet, and the Rebel Alliance a smaller fleet.
However someone pointed out that, if that’s true, why are there so many different types of aliens? At the time I didn’t have an answer.
Today my girlfriend suggested that there might be a single species of alien which changes its shape.
This, I suggest, is a genre-appropriate solution. It also explains why there are so many more types of aliens in the cantina scene than anywhere else: these shape-changing aliens usually prefer to look like humans, but because they lose control when drunk they’re more likely to take on different forms.
Alternatively, it could be (GF’s suggestion) a species of alien that has a very complicated life-cycle, or (my suggestion) a Barsoom-like planet that has lots of different intelligent species.