Of course you can go for a town theme after creating it! Your town is yours
As for suggestions, all of the above are great ideas, and I'll recommend some villagers that could fit the space theme.
-Octavian: His house is space themed, he could be like an alien octopus thingy.
-Ruby: Not only her house is space themed, but she's basically the lunar rabbit.
-Julian: A celestial unicorn, of course.
-Soleil: A solar hamster, could be the counterpart to Ruby.
-Ribbot: a space robot frog? Yes!
-Static: He could be some sort of plasma or electric squirrel?
-Coco: She could be an alien or some kind of asteroid related to gyroids
-Mira: She's based off Sailor Venus so she's kind of an astro rabbit
-Boomer: An interstellar penguin pilot?
-Astrid: She has stars all over her body
-Skye: The sky wolf, maybe could work
There's probably a couple more, but those are all who came to mind
Good luck with your town!