They leave the adult shrimp alone but if they can get their "hands" on the babies they will eat each one they find.A lot of people buy Neon Tetras from me as shrimp counterparts. The tetras leave the shrimp alone and they are cool fish to look at.
Least killifish, Heterandria formosa, are live bearers like guppies and mollies and not self replicating.I have Least Killies with mone and they are just too small to pose a threat to adults or babys that get any size at all on them. They also tend to hang out at the top of the tank, so there again your shrimp will not interact with them often. Least Killies are also self replicating. If you are interested in getting some just p.m. me.