1.- You can create a truly global glossary for a whole moodle site. Create that glossary on your moodle site's front page. Have you tried it?
2.- Not sure I understand what you mean by "pull entries from specific categories etc."
Please keep in mind that glossaries in Moodle serve 2 functions:
a) a glossary is a resource which can be browsed by the users, added to and commented upon (if the necessary permissions are granted to specific users);
b) if that feature is implemented, then autolinks to glossary entries are dynamically generated for a number of other resources (HTML pages, uploaded HTML pages, quizzes & lessons, forums, etc.
For me feature (b) is the most useful in my teaching.
Joseph