Lets clear something up.  A parent portal really only has anything to do w/ a domain name, same thing with child portal.  So wether you have 3 parent portals and 3 child portals or 1 parent portal and 5 child portals it doesn't change how modules behave.

There can be multiple instances of a forum on the same site.  The Forum module does not share data across portals.  There are a couple situations we need to investiage further but I am pretty sure most of them are already cleared up.