Being that Bull sharks inhabit the Amazon, could they prey on piranhas and electric eels?

Answer 1

Although I was tempted to go into a big, long answer, I've thought better of it.

Yes, bull sharks inhabit the Amazon and a great many other freshwater rivers, and if they found one, yes they could eat a piranah or an eletric eel, although I would guess that trying to eat such an eel is a mistake thet the shark would only make once.

Answer 2

They could and no doubt do eat piranhas but I think the eels are usually found in low oxygen stagnant water. They get most of their oxygen from the air. Sharks are very sensitive to electric signals and would probably stay clear of the eels.