I don't know if a 55 would be big enough for a few, maybe 1, 2 at the most. My dad wanted pirhannas, but he wasn't ready to house them in the corect size tank. I had to talk him out of it.
50 gallons for one fish, 20 gallons for every additional fish is what I would go by but I know the absolute minimum is 50 for one and you'll either have to get a single one or a trio. A pair will fight.
It would also be helpful to know where you live...
I have seen some pirrahna sold at the Living Sea Aquarium in Chicago (Park Ridge actually). Never looked at prices or whatnot, can't house them and am moreinterested in community fish in general.