Generally speaking, when adding a subcategory to the fiction portal (archive), we prefer not to create the subcategory without someone having something to put IN to it once it's been done.
The ten stories thing as a breakpoint was something I'd suggested for pair specific subcategories, which is something a bit different. Structure there would be as follows (this is eventually how it will go)
archive >category>pairing type sub category>specific pair category
For example, using the HP archive as the example; you'd have
http://hp.adult-fanfiction.org>main category listing>het>Harry x Ginny
We can't do this yet (the software itself does not support unlimited sublevels, which is necessary), and it certainly will clean things up as well as simplify them.
If you're asking how things are added in the forum, that's a bit different. For the fanfiction section, there are forum categories for each archive subdomain, with discussion forums as subforums. In some where they have 200+ stories (i.e. the subdomains with many, many title categories such as comics, books, and anime), then those title categories get their own subset of forums for discussion as well. I'm still doing all that, by the way, so there will be some additions yet.