Possibly a single black piranha, you can't keep them together at all. Depends on the dimensions of the tank really. You are looking at incredibly shy, lazy, inactive (and personally I think very boring!) species of fish that's going to get a foot long and need vast amount of filtration.
I kept 9 red bellies in a 150g tank they were about 5 inch or so and they were inactive boring and hard to maaintain as the water levels were high due to the waste they exude so lots of maintenance needed