BuddyPress.org

If I understand your requirements right, I wouldn’t use multisite for this. I also wouldn’t use separate installs.

Considering you just need a main site area and a BuddyPress area, I would aim to keep things as simple as possible and address this problem with a custom theme. You can add a /your-theme/buddypress/ folder which contains all of your BuddyPress templates. Then it’s just a case of styling both the main area and your BuddyPress pages to look consistent.