Would that screw up SEO if we forwarded that domain to www.mydomain.com/social or social.mydomain.com and built links to that?
If it's a new subdomain without link juice, just transfer the files to a folder.
I don't know if this will help out at all, but I'm putting it in here to demonstrate different softwares.
1) http://www.shipoverseas.com/ MAGENTO
2) http://www.shipoverseas.com/blog/ WORDPRESS
These are two completely different softwares, but the menus are the same. As you stated, you have different menus. WHY? What softwares are you using and why not just "bridge" them with a plugin?
1) Do you think it would be professional to have the social site there with a different look?
NO. I think more people would stop and wonder if they are on the same website.
I am looking forward to your response and would like to help