I don't have piranhas, so take anything I say with a grain of salt (especially if a margarita comes with it

), but I think you can never have too much variety. Still, you are offering them a pretty decent variety. I know that a classroom in my elementary school fed their piranha nightcrawlers.
As much as I would be unable to deal with the way piranha's eat their food (which is why I doubt I'll ever keep them), I think that they should have live food periodically (not feeder fish, they're rife with disease and not all that nutritious). At the very least, use a pair of forceps to "play" with them, moving their food around in the water so they have to "hunt" it.
Also, I bet they would enjoy a bit of red meat periodically. While attacks on terrestrial animals don't happen very often, they do occur.