Exodons are kept in large shoals to prevent killing each other. 10-12 is the bottom line for keeping them, most aquarists would suggest keeping at least 15 - 20 of them, but that won't fit your tank.
If you want some predatory fish for a 55g, I'd suggest 3 red bellied piranhas. Over time you may be able to introduce 2 more red bellies. Keep them well fed and you should have no problems.