Diet should be a mix of A good chiclid stick or pellet (hikari bio gold, Tetra chiclid sticks) Kril or market shirmp, frozen fish fillets (talapia of what ever is cheep at the time. And if you wish add in some live food.
Tank maintenance is not hard as long as you keep up with your water changes. One big 50-65% change a week should be fine as long as you have good filtation. (a good rule of tumb is 10Xs turnover. So a 55gal tank should be in the ballpark of 300gal turnover an hour.) This filtration rate is debatable however if you keep up with the water changes. More water changes less filtration needed
Personally If I ever do another Piranha tank it will be a clean bottom. no sand or gravel. It makes cleaning ALOT faster.
5 in a 55 fr a year should be fine but know this a good well feed RBP with good water can grow 7"+ in one year. then somewhere in the ballpark of an inch a year afte that till they are 10" (rarely over 10" but it does happen)
Also RBPs are pretty messy so the filtration and water changes are top on the list of things to never forget to do.