It would help to know what species or variety of killi you're discussing since it's a fairly large and diverse group of fish we're talking about here. Also, it wouldn't hurt to share with us the size of your tank, whether it's planted or not, things like that.
I don't think black water extract is going to hurt any fish, though it can lower your pH and hardness levels, which might not be a good thing for fish that prefer hard and alkaline water. That's certainly not going to be a problem for your platy or tetras and many other of the more commonly available freshwater fish.
Regarding the killifish, you really do need to determine what kind you've got or are planning to get. There are some species that you almost certainly can get away with having just males in your tank. There are other species where having more than one male in a tank that's too small will lead to very aggressive fighting and the death of all but one fish.