Can followers interact with each other? Like form relationships with them? Or is it strictly just the protagonists who can do things with them?
Everyone from natives to animals can form relationships. Native women can even have sex and reproduce to help you get more people to help around your camp. Just make sure they don't have conflicting personalities, or else a fight or sexual assault might break out.
Their personality, likes, and dislikes can be checked in the status screen, so it's best to assemble allies who at least won't hate each other. As long as they don't dislike each other's personalities, they will get along.
One thing I noticed is that animals also have personality preferences, but these do not display. You kind of just gotta do some testing and see what they like and dislike.
From my experience, relationships only for between individuals with overlapping territory, but don't quote me on that.