Caribe piranha luckily for you are not Serrasalmus; those are another whole genus of piranha all of which look very different to caribe and red belly. Caribe belong to the genus Pygocentrus which can be kept in a school. However, caribe in my experience are the most aggressive pygo. But it can be done. You can add more caribe to the tank, you can red bellies of the same size. Pygos can also be kept with exodons and dithers.
If in the future you are considering a Serrasalmus, they are solitary because of their aggression. You might get away a pleco or tetras, but most times it's a ticking time bomb. Not a good idea to play Russian Roulette with them.
Just for an added FYI, both Pygocentrus and Serrasalmus belong in the same family serrasalmidae which also includes the vegetarian piranhas (although they are not marketed as piranhas), Myleus, Metynnis, Mylssomma, colossoma, and myloplus.