Zigx, I think you are following a logical progression - start with a general topic, and add new forums when you have quite a few posts about that topic in other forums. From what I can tell, most of the topics here have evolved in exactly that way.
Sometimes, of course, it makes sense to start a topic in anticipation of major developments in an area, or in order to set a new direction for your forum.
You'll always find some members, particularly new ones, who misplace their posts. Not much you can do about them except move them and try to train the posters.
I don't think it's a great idea to start too many new topics - you'll have to live with some posts that just don't fit perfectly in their current categories.