If you want shoal go for caribe, a lot less skiddish than natts. My natts will eat in front of me and rip things apart as soon as it hits the tank, but I have had them since they were nickel size, and it took them a couple months before they would do so.
If you want a solo get an elong, or a manuelli. From what I have heard mannuellis are very agressive and very fun to keep.
I have an elong and he is by far my favorite fish, agressive and just mean looking.
If you want large go with a rhom, but if you get a juvy, do not expect it to get much larger than 10 inches or so and it will take years and years to reach this size.
If you want a monster rhom you will have to buy it already at 13, 14 ++ inches.
You said you would probably devote the tank to the p's, do not get them unless you can devote the tank to the piranhas, they wont be able to cohab with anything else.