So as to get a single index of both internal posts and external links on the same or related topic/subject matter, is it possible to combine post categories and link categories? I would like to be able to drill down through a single hierarchy, and find all collected information on the topic, whether external or internal to the site. Thanks.
You can register a custom taxonomy for more than one object type:
register_taxonomy( 'topic', array( 'post', 'link' ), /* more arguments */ );
But be aware that the link manager will be removed in 3.5, so it might be better to use a custom post type for links. You can register a shared taxonomy for that too.