I would most def not keep a school of macs in a 55 due to their serra nature. No mind if they are small or not, they'll pick each other to death.
Likewise, dilobom, you should looking into picking up another Pygo in order to curb aggression. Usually if you have two, one will fight for dominance. If not, that is excellent it's working. Secondly, never, ever, ever keep anything with piranhas unless you are well aware it'll become a meal. Just because it works for a day, week, or even a couple months, does not mean it'll stay that way.
To the original poster: There are many species of serra that would do absolutely excellent in that tank.
http://www.aquascapeonline.com/aquatic-live-stock-fresh-water-fish-piranhas/
^^I'm not posting this saying you have to buy from there, but rather it has amazing pictures.
Stay away from all rhoms, eventually it'll outgrow that tank, unless of course you plan on upgrading.
Piraya and Caribe will eventually get to big (even 3 in a 55)
Any other ones, you should be fine. Of course, you could always divide the tank and have two small spilos or something for now!