I'd go for the cheaper option at 27", no way I'd shell out another few grand for a couple inches in height. The tank you listed that I mentioned is about 405 gallons, pretty big! I'd get two dozen baby RBP (inexpensive at $5 a piece rather than $50 a piece. They'll grow pretty quickly, you'll lose a few, but if you don't you can always trade some in. It's easier to start with a large group than to add new specimens to an established piranha tank. Usually the new guys get teamed up on. Growing up together definitely helps with aggression too.