Like others have already said if you're breeding then conditions must be perfect. I saw your explanation so I won't harp on it but like Andy said, if you plan to breed endlers (or guppys) then they should be the only species present. I know it was temporary but look at the implications now. Just make sure if this is a goal to prioritize for it. I successfully bred guppies for profit for two years and made decent money. The main factors are fry survival rate and having healthy females able to produce large batches of fry. For what its worth, even at that small size those female endlers could've already been pregnant, they can breed extremely young.
Pm me if you want some extra info.
