The issue is not in finding a category which is less than 20 articles and tagging for deletion. The issue is in taking the immediate actions of both blanking the category AND de-tagging all the existing pages like you did.
Existing Categories—as opposed to new categories—are given time for pages to be added. Especially for a show such as Kim Possible where there ARE more than the needed number available.
Doubly so for being short by only ONE article.
In fact, the helpful thing to have done would have been to add at least one article.
Unfortunately you have not. In fact you're undoing an admin's cleanup work which is a no-no. Now I have to cleanup the page again. This time I'll take care of the naming of the group's pirates whiel I'm @ it.
I'm afraid I can't since my time here is almost up for I have to go to work today & I won't be back until 7pm. I think one of the available admins here might be able to help you. Also, by watchin' the whole movie Igor from this website might be of good aide for you.