An absolutely adorable fish for that size are Endler's Livebearer.
They would be great with some Java Moss (if you want to save some of the fry), and some hardier plants such as Anubia, Hygrophilia, Java Fern or Pennywort.
You could go for 10 Endlers and just let them multiply, maybe add 3 oto cats for algae control.
Another idea would be killifish, absolutely underrated fish IMO.