If what you're trying to accomplish is being done here: http://xenforo.com/community/resources/ (with multiple categories handling different types of resources), then yes. 1 license per site. As Mike mentioned, you can't really install more than one instance per forum, anyway.
Its in attachment. Very simple add-on created in 5 minutes that adds 2 tabs: one with dropdown menu, one without. Edit Listener.php to change those tabs. Tabs are basic php array. I've left comments in php code to explain what each line means.
Edit template 'extra_tabs' to change content of tab with dropdown menu. Create more templates like that with different names if you want more than 1 tab with dropdown menus. Change 'linksTemplate' in tab's data in Listener.php to correct template name.