You can do a single piranha in a 55g. Although, they are a shoaling fish by nature and run in fairly large groups. That being said, eventually I would get at least a 120g and put them in a group as they are intended.
You could get away with running HOB
's on your 55 with regular weekly water changes and gravel vacs if you are worried about the mess they make.
If you are set on feeder fish I would get a separate 20g long tank and stock it heavily with feeder guppies. I'd imagine an adult piranha would consume several a day. Might get spendy plus you have to worry about your piranha contracting a possible disease from your feeder fish. I guess that's a risk you take if you wish to watch it hunt it's food. There's several frozen foods you can feed such as prawns, fish and beef heart. A single piranha can eat up to 1/8th of there body weight daily!! An adult can weigh between 4-8 pounds.
I once was set on keeping bass in an aquarium until I found out how much they eat and the requirements they needed for me to keep them and thrive. I wouldn't mind following through on the idea but it requires a very large tank and filtration system. I think you will find you will also be in that same boat.