The categories on a forum can be a critical issue. Have you ever seen a forum with more categories than it has members? I've seen a few like that. You do need a certain level of traffic before you start a forum.
Even with traffic, you have to watch about watering down the posts with too many categories. Many forums spread themselves too thin in this regard and the forum, even with a decent number of posts, looks like a ghost town.
Build traffic, start the forum with a few categories, build more traffic, and expand the categories. That would be my approach.