All in all the advice Zed is giving isn't bad. It may be a bit too specific of an interest group to draw much attention.
That said, I don't think sub topics are a problem. We have an Avatar thread for "either series" and one for Legend of Korra and it hasn't been a problem.
We have the no duplicate threads rule just because we're still pretty small and we don't wan to split the user base further. In this, I can see where Zed is coming from. As I said earlier this is a sub topic so I think it falls a little differently.
It's not like I intend to remove gay content from the AWG thread but if someone wants a thread with just gay AWG then it makes no difference to me.
If either of them bump it's a hit for the page and at least one of them is a distinct separate interest group.
As Zed warned, OP should beware that if you get your interest group too specific you probably won't get many hits on your thread. I would know, I've made plenty of threads that haven't received much feedback. Now if you'll excuse me I'm going to make another.