i feed all my piranhas pellets and raw shrimp as the main part of their diet and i mix it up with tilapia, catfish, frozen krill, and silversides. beefheart is ok sometimes, but mammalian meat should make up a very small part of their diet, no more than once a month if you want healthy fish. getting your piranhas on a high quality pellet will be the best thing that you can do, it can sometimes test your patience, but it's much easier when they are small and it's well worth the hassle. getting them to eat non-live food should be very easy when they are small, they will eat just about anything, just make sure that once you stop feeding live, you don't go back to it.
lighting really doesn't matter, your fish will get used to any light you put over the tank in a short time. i have 8 piranha tanks with all different types of lighting, including some very bright lighting, providing lots of cover to create natural shade is much better than dimming the lights.
i would add a powerhead and point it towards the front of your tank to get some of that current off the back, just make sure that your filter outputs as well as your powerhead are all facing the same direction so that you don't have multiple currents swirling around in your tank.
don't put anything else with them, it's just not worth the risk. some people put plecos in with theirs and sometimes it works, but they create alot of waste.
good luck with everything, if you have anymore questions, feel free to ask.
I will post a pic of the tank so I can get our guys thoughts of the setup.