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It seems to be that some people get it to work in a community and some just wake up to dead fish and eventually see their killis stalking other fish. Although your stock seems like it'll be fine so you don't have to worry. You may observe males fighting too but good luck I hope it works out for you. They're easily my favourite fish
I'm not a killifish expert but I suppose some epiplatys annulatus could be an option. I have heard that they can be delicate but once they acclimate, they should be fine.