From memory because piranhas generally have a protien heavy diet and meats etc will foul the water quickly you are better off doing a tank clean after each feeding, removing (usually via a siphon) any uneaten food. This is on top of your weekly water changes.
Also you need heavy filteration to take care of the large amount of waste these fish produce, a good rule of thumb is double the filteration that other fish their size might otherwise need.
Please also keep in mind that Piranhas will also eat some vegetable matter, there for also offer occasionally greens such as zucchini, spinach and peas.
I would not consider adding any "clean up" fish since there really is no real creature that cleans up totally after another species. Snails may work with some cleaning and not become food for the RBP., however generally snails are going to be mostly interested in vegetable type foods over lots of meaty left overs.