I have three of them in a 55...thats how much I like them.
Never shy, always running around (including running up your arm to clean you when your hand is in the tank), clean ALL my fish and my eel. Great scavengers, always pregnant...whats not to like!
Mated pair - it is my understanding that they are born hermaphrodites. When you keep them in a little group (2-3 is meant to be a good numebr) then they "develop" sexes. So basically, if you want a mated pair, just keep more than one of them, and they will figure it out themselves!
Stewart