I really like your ideas...
...The thing that bugs me though is the cap on the amount of players that can be thieves. Other than that, I think it's all pretty cool. At the current state of the game, it probably wouldn't be too bad. But by the time all of this could be implemented, I would think that number might be too limiting. Going around deleting old accounts that just so happen to have a thief in it because the cap has been reached, or killing off bad thieves, or whatever... doesn't seem fair. From my knowledge of the ingame law, you would only get banished or something if you stole... you only get perma death if you perma kill, or something really bad like that. I could be wrong, lol.
Also, how would you tell if someone was exactly a thief? The way things are now, it doesn't really give out exact classes. You could be a bit of this and a bit of that... is it based just on the skill they are advancing? An assassin might use some of the same skills, so it's hard to place a cap on thieves without hindering other types of people. Unless it was only on the pickpocket type skills, I guess... but even then, people who didn't think of themselves as a complete thief might still have it for reasons of their own... but I suppose you trying to make the skill hard to learn would stop most of that. Or at least, it would stop a random person becoming 'uber' at it, anyway.
So, yeah, let me re put down my thoughts here, I'm getting a bit muddled...
It seems like it would be hard to cap the amount of thieves in the first place, since it's hard to tell who is a thief. If a person try's to train in a skill, say, pickpocketing, they would get the message: Ooops! Sorry, there are already too many thieves in the game, heheh! Too bad!
Or if a person wanted to make their character have a specific background according to mechanics, and it just so happens that they get one point in pickpocketing... they suddenly have to rearrange everything because they get a similar message.
You get my point?
So basically, you want to make a thief hard to train and hard to keep? =p