I've been a part of many forums and I can tell you that this idea won't work. While it's good in theory, but if you get too many sub-forums things become chaotic. People don't know which forum a topic belongs in, posters are beating up on each other for posting in the incorrect forum, it's tough to find the info you're looking for, etc., etc.
We tried a similar setup in a forum I moderated and it just didn't work. What we found was that a few of the sub-forums dominated with the majority of posts and the rest were pretty much abandoned. Ultimately the community will tell you how your forum structure needs to be setup.