Good morning from Texas! My family set-up is: my father, my mother (his wife), his “main” girlfriend and 2 “secondary” girlfriends which get changed usually every 2-3 years. Yes, we all live together in the same house - though it’s a big house - so it doesn’t feel crowded. It’s not a religious thing at all, we’re atheists.
Yes, I’m the only child. Yes. I wasn’t insulated at all, it was pretty much an integral part of my life. As for ground rules: mom (his wife) was the only one allowed to parent me, I called the other women by their first name, mom and dad always shared a bedroom, with the main girlfriend joining most of the time, but she has her separate bedroom. The “temporary” girlfriends as I call them have their own bedroom and as far as I can remember never slept in the master bedroom. Overall the whole thing is very hierarchical looking at it now. I’m on good terms with everybody.