In general,
P. alleni should be housed individually.
They are quite aggressive, and any attempt at housing them together---particularly in smaller (under 4 ft.) tanks---is very risky.
For those of you expressing interest in getting juvies from my brood...
I'll be putting up a thread in the Buy/Sell/Trade section with further details as they get closer to shippable size. Also, feel free to send a PM at any time, and I will do my best to accommodate everyone's requests.
Gerald...yup, a 15 gal. for a single
alleni would be absolutely fine.
BV