Well you could do a male guppies or male endlers, two of each but not together. They'll spar but they won't kill each other generally. Or you can get one Killifish, I love Killifish and think they would make a good choice; they do like well established tanks and the smaller one's bio-loads are relatively small. So my first pic would definitely be just one Killifish. They're water requirements are very different to each genus so make sure you look into that as well. I can help you a little for finding the right species since there are thousands, I'm only an amateur with Killis but I can at least get you on the right path lol.
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen