Multisite installation questions:

Can each site have it's own unique forum?

With:

1. Shared user/subscriber base for the forums/sub-forums of every site included in the composition of the multisite.

2. Individual/unique user/subscriber levels/roles for any of sites included in the composition of the multisite's forums/subforums.

3. A user may be a designated any role, such as admin/moderator/etc, for one, or more, forums/sub-forums for one, or more, of the sites that are included in the multisite.

Basically each site included in the multisite would have it's own SimpleForum with forums, sub-forums, post rankings/votes and user/subscriber roles unique to each site with all the sites sharing a common user base. (users/subscribers are able to move move between "site forums" with a common username/password based on subscription/permission).

WordPress:

Each site within the multisite is treated as a library containing a unique subject with many categories, sub-categories, sub-sub-categories, themes, sub-themes and sub-sub-themes. (A hugh multisite that will grow to contain many hundreds of sites)

Individual forums would replace the comments of posts to any of the site's, included in the multisite, categories, sub-categories, sub-sub-categories, themes, sub-themes and sub-sub-themes.