I think this is what Batman means when he tells Dent that Schiff is a diagnosed paranoid schizophrenic and a former Arkham patient: the sort of mind the Joker attracts. For those following the virals, I think a good idea was given there of the way the Joker might attract his followers: clever newspaper ads, games and tests. He doesn't want standard thugs who are just in it for the money, and who might figure him out too quickly and decide he's expendable before he decides they are. He wants guys who'll do it largely for the game. (That said, they do seem to have to have a lot of the standard criminal skills: given the precision of the bank robbery, the chase involving the van carrying Dent, and the hostage-taking in the building overlooking the ferries, he obviously wants guys who know which end of a gun to fire, how to drive, how to set off explosives, etc.) Whether he has some sort of hideout and inner circle of bodyguards and drivers is never really made clear, but I'd tend to think he alerts the guys he needs for jobs through some sort of public forum like the newspaper classifieds, so that they never find out too much about it. Schiff's protest that he knows nothing is probably true.