Creating Custom Menu Links with Social Theme

How do I set up custom menu links with the Social theme. Here’s what I’m trying to do, I am creating a social network that will be a combo of a video section, blog section, a place to post daily activity, fan pages for bands (included will be ability to sell merchandise from there page), etc. What I would like to have is seperate themes for each section, but all coming from the main theme (Social). For example, maybe use some video theme similiar to youtube for the video section. I have set each section up on different sites and would like to use custom links to navigate to each. I also would like there to be a way that once a user is logged into the main site they don’t have to log in again when using the links that link to the different sites. Maybe I’m going about this the wrong way, but all help is greatly appreciative. Thanks