I haven't had a chance to keep any caribe's yet but I heard that as far as shoaling P's they can be the most aggressive, because piraya's take a little bit longer to lose their skittishness because caribe's mature faster. So I guess short term your best bet is caribe's, long term definitely piraya's. I had an elong when I used to live in Jersey, and honestly he was a wuss (always hiding in the plants and driftwood, surprised to find out their an ambush predator, they lurk and dash out at food once you drop it in there), but a friend of mine had an elong that would charge you if you came near the tank. Kept rbp's before which were really skittish, but were very predatory towards anything in the tank.
Since you plan on putting them in a pond personally I'd go with a shoal (caribe's or piraya's), you may get bored with a solitary fish. You can find them in your country, you'll just have to go straight to an importer rather than an lfs, but you just have to be willing to pay 3x the price elsewhere. BTW, every piranha is aggressive if it's hungry ^_^ sometimes I think my friend used to starve his elong in advance if he knew anyone was coming over just for the theatrics, never tried that stunt with my elong though coz I just didn't see the point. coz if it really had that kind of bad attitude off top I wouldn't need to starve it to witness it.