What about killifish? They have tons of very cool varieties and they are generally pretty peaceful. Problem is that most don't live very long, maybe a year to 18 months max for most of them...but that'd be pretty good since you were thinking of switching to SW
and using this as a sump eventually anyways.
You could also look into guppies or Endlers. They are small so you could have a pretty good amount and they are very colorful with long flowing fins, but they reproduce faster than rabbits so you'd want somewhere for the fry to go if the parents don't end up eating them all.
Neon or cardinal tetras are my favorite. Those with some German blue rams would look great, with maybe some kuhli loaches for the bottom...
There really are a ton of great choices!
Look online, see what you like best, then come back and ask if they'd work together. There are tons of experienced people here to help you in deciding!