I don't know, maybe I've just been lucky in the groups I've been in. We've always aimed for diversity. Part of it is that we're always conscious of covering all the major skill sets. One combat wombat, one skill monkey, one social monster, one reality-defying freak, etc. I think it would be pretty easy to just say, "Hey, people, try to get a range of things going," rather than any hard and fast rules. After all, I find that there are LOTS of excellent examples of character types in the books (and the TV series, for that matter) that aren't wizards at all.
As for balancing the players, that comes down to two things. One, make sure that when abilities have disadvantages, you enforce them. Make your wizard actually sit out of the action when he's brewing potions. Occasionally, make him regret using his evocation at full power. Two, make sure that when characters have advantages, opportunities come up in the game to use them. If a guy makes an IT worker whose advantages include significant disposable income and computer skills, make sure that your stories offer up ways to use money and/or hacking to solve the problem. Then, it doesn't really matter that the wizard might be, on some giant cosmic scale, "better" than the IT worker. Both characters are contributing equally to the story.