How exciting! And a great tank size. I did a cool pea puffer tank in a 20, but that's a lot of work. I always like long tanks with just a big shoal of a lot of fish - like 20 tetras, or 30 chili rasboras - plus maybe a show stopper. You could maybe keep a pair of Apistogramma Cacatouides. Can't go wrong adding cherry shrimp or something of the like to most tanks (although I do know some killis will attack the shrimp).
Sounds like you want a tropical community tank. Which would be great - they're easy, low maintenance, beautiful, and have a lot of options. Have you considered a dwarf gourami? I find a dwarf gourami with a small shoal of fish and then maybe a cleanup crew (depending on how your stock load is looking) would be great.
Don't have much experience with killifish unfortunately, and I'm not the one to ask about livebearers. But those are the thoughts that come into my head
What's almost even more exciting is what plants you might put in it