there are many aquarium fish that are tougher than a piranha such as a pacu, wolf fish, red devil, wolf cichlid etc. just to name a few but those aren't even the deadliest fish out there.
You can mixed any pygo species of piranha together. make sure it in within size.
for Serrasalmus species you can keep spilo and geryi for experience keeper. other then this it's a risk, burning your money, wast time, not recommend .
There's a thread "sticky" on top of this forum listing possible tank mates. It's really a toss up in my experience. You only got a chance if you try mixing pygos with something else. Sometimes they don't do anything and sometimes they go into a feeding frenzy.
Aggression can be defined in many ways when it comes to fish keeping. One way is, how viscously a fish eats. If that is a way one defines aggression in a fish they are looking to keep then piranhas fit the bill perfectly!