Regarding PHP-Nuke, I highly do not recommend that software since it usually requires pay and well.. ridden with bugs as far as I'm concerned.
Anyway, I looked at a few posts for multi-forums sharing the same user table in the database, but I haven't found too much as nothing out right now is stable.
As far as linking to the other forums go, you'll have to manually edit the HTML file in the overall_header file and manually add the links yourself. I don't know how your forum looks or anything, so that's about as much as I can suggest about the matter.
For portals, that's up to you. I'm currently messing around with phpBB3 Portal
and it works quite well, easy to customize and such. Again, up to you what you want to do for your forum.