I keep killifish and bettas. I keep West African killifish, and many of these species are extremely bright coloured, fairly boisterous and quite active fish.
I don't think they make ideal tankmates for bettas. I think your betta is either going to be overwhelmed by the larger more aggressive species, or is going to chase and harass some of the smaller more shy species.
I have no clue as to American native killifish being ideal candidates for a betta tank, but the West African killies I have experience with tend to do best in peaceful communities or species only tanks.