I was at a LPS the other day.and overheard this
Patron: Can I put other fish in with this Piranha? (points to a Piranha)
Employee: If you put them in as babies, they should get used to each other
So I walked over
Me: You can put what ever you want in with it, but as soon as he gets hungry or has a bad day, the other fish will be gone.
It turned out the employee was new, and had alot to leqrn. On the bright side, he was eager to learn.