Killifish isn't a scientific term, but a hobby one that's convenient for us. In all cases but one I know of (a tiny American livebearer called the 'least killifish') the fish are classified right by the hobby. Pupfish are an evolutionary sub group within the egg laying toothed carps (killies), in Cyprinidontidae. They're higher bodied and slightly dog-faced, in a bulldog way.
They're also North American. Flagfish are from Florida and are territorial egg-layers. Keeping them is completely different from African or South American killies. They like slow moving, weed choked hardwater. They're all around the drain off ditches around Orlando Airport. I almost had my rental car stolen learning that.
I have never seen one frolic, more like an alcoholic on the morning after. Mine have been kind of grouchy, although I quite liked them. They're tough little fish. Personally, I would never keep them in a community. They are for single species weedy tanks.
In one stream about twenty feet wide, they were in weed beds in the middle where I spotted them but couldn't catch them. Heterandria formosa, the least killifish from above we along the shallow, with bluefin killies in large shoals, all avoiding the weed clumps. There were also a few juvenile mollies, Poecilia latipinna, the American sailfins around.
A guy I gave some of the formosa/least killifish livebearers I caught there to gave me some of their descendants back a couple of weeks ago. I was there in 2015, so they are quite a few generations along. But I don't have flagfish. My water's too soft.
