Re: Few questions..
I just set up a bbpress forum and use the WP user database. It works well though plugins are needed and some hackery of bb-config is required (see here.
I use the private forums plugin. There is another plugin that blocks all access to the site. You could always make ma minor mod to bbpress source to force login and not rely on a plugin.
I’ve seen some discussion on WP about BBpress. I believe they changed the way they handle tags in order to not step on bbpress.