Peppermint shrimp, depending on the species live in small groups; sometimes in large aggregations. I normally find 2-6 in ledges (really small cracks), sometimes hundreds in large overheads but I question if it's a real "group" or a bunch of small groups moving around. Cleaner shrimp pair bond, although loosely; you can put any two together pretty much at any time and get a pair, the pair will kill other cleaner shrimp that are in the tank growing up with them or that you newly introduce. You can do two fire shrimp, two cleaner shrimp and a few peppermint shrimp. But don't put more than two fire shrimp or two cleaner shrimp in a tank.