yes piranha eat fish, and with many piranha it is their priamry diet, but not for all... some will eat vegetation, or insects, and even birds
you are correct in saying that only feeding fish is not good, as like all animals fish need a varied diet. but it really depends on the fish itself, and what they eat in the wild...
back to piranha, let's take the most common (the Red Bellied Piranha) they're primary diet is other fish/fins. so feeding red bellies a diet of fish, and only fish is an excellent diet for them, as long as you're not just feeding them the same kind of fish. when i had mine, i would feed them salmon, haddock, smelts, and every once in a while a few goldfish. and they were in perfect health, and grew quite fast! and as you see there is a mixture of FW and SW fish there
but of course this diet would not be sufficient to all fish
as i've read a lot of cichlids require a mixtuer of fish and vegetation, so obvioulsy a diet of guppies would not work, but feeding them every once in a while is okay!