Does anyone know anything about keeping killifish in a tropical fish tank?

It depends on what kind of killifish that you have. The Florida flag fish is a type of killifish and they will usually protect their eggs. Then there ore those killifish that grow and mature in less than 2 months because their water will dry up in just a few months, most of these killifish have eggs that will hatch at different times (once they are wet), this will insure that if just a little rain shows up the first ones will not be the last ones. and there ore some like the golden wonder killifish that can be bred in the same tank as the Yong are growing as long as you have enough live aquatic plants growing in for the babies to hide in and find infusoria to eat, (I know because it has happened to me a few times, I did not even know that they had spawned). Other killifish will spawn on plants (killifish breeders use "spawning mops" to catch the eggs where they are then partially dried out and stored for a length of time (depends on the species of killifish as to how long of a time that they leave them before they put them in water to hatch).