What species, remember your going to want schools, big schools and separate tanks. This way you can hospitalize any damaged fish. Be sure to do plenty of research before getting any. But again, what species of Piranha?
Also as a note do a fishless cycle and hold it for a while as all members of the same group need to be introduced at the same time. Anything put in after is fair game to them. Yes I would say at least a 125 or 6' tank will be minimum as my LFS has 8 in a 55 and they don't move very often. Lastly make sure they are legal to sell, own and purchase in your local area.
I had been looking into them as a possible for a large tank, but after reading up on people that keep them I found that they are a VERY boring fish to watch, usually just kinda hanging there in the water.
Also as to the Laws in your area, make sure to read the law fully, I know there are a lot of fish up here in new england that can be bought in some states and not others, HOWEVER it is not illegal for you to buy it in the other state and drive it across into the state that they are illegal to sell in. The only time that comes into play is if you OWN a fish store and sell as a business. Other than that you just can't buy them. Kinda weird how the laws get worded sometimes but hey if I can still have the fish I'm all for it.
I would also agree that you need AT LEAST a 125gal tank the bigger the better, these fish can take up large territories and travel in very large groups most of the time.
Ah yes, adding any fish after the initial stocking you normally need to go through some pretty extensive breaking up of the tank and group. We had a guy on the betta forum I'm on who was BIG on piranha, he said that he added 2 more to his school after 3 months, they were cannibalized and they school ate one of its original members in the frenzy of it all. They are intresting to me lol. They have beautiful color and charm. Even if they remain still.
My LFS has a 150 gallon in the back room with red bellies. Its a Amazonian bio type very cool tank but the fish never move, and when you put in a feeder fish most of the time they don't even care about it they will nibble every now and then but.... not much action. They love blood worms and breed like crazy.
If you are looking for a cool aggressive shoaling fish look at the exodon tetra. IMO one of the coolest shoaling fish there is
Many piranha keepers will actually recommend getting your fish onto dead meat. Just because unless your breeding your own feeders you it can be risky with store bought, so don't get a fish just because of its dietary habits