You can set Safari's preferences to check for updates as often as every half hour and whether it should update things in the Bookmarks Bar, Bookmarks Menu, or both.
As for getting an RSS feed of just a specific thread, that depends entirely on what's referenced in the header of the page in question. If the header does not specify an RSS feed for that particular thread, then no. So, I guess what I'm saying is "it's up to the forum software in question."