elongatus get to about 9-10 inch at a maximum or a sanchezi they get 8 inch and both solitary, although not a true piranha wimple piranha are an option, not sure on schooling them in a 55 tho. exodons are an option if you want piranha like fish as a school.
Yes, the cryptoheros family of fish, like the Honduran Red Point and others would be great in a 55. I think you could have a few grow up in there nicely. I have nanoluteus, panamensis, and septemfasciatus all of which are colorful and very personable CA cichlids that will grow up to 5 or 6 inches, but showing all the signs of any of my other wet pets (Oscars and severums).