Killifish are gorgeous! Not hard to keep at all. My suggestions from personal experience are:
1. No small fish like neons
2. No small-ish fish with longer fins - my killifish beat up my long-finned danios tails, but did not nip at their bodies.
3. They spend most of their time at the surface, so the surface should not be crowded with other surface dwellers.
4. Keep them fed well with a variety of foods like shrimp, krill, live foods, frozen foods.
5. They are jumpers so watch them when the top is open!