and their SO’s have very different expectations on each other, and those expectation (to me) are easier to work with than the expectations on live-in partners.
If you mean that solo poly people are more likely to practice a relaxed, trust based form of ethical non monogamy opposed to couples who are more likely to feel the need to protect themselves from betrayal with rules and protocols, you're absolutely right. Yes, I am solo right now because, as I said, I don't share practical entanglements with anyone. I don't plan to always be solo, but this won't mean that I suddenly insert rules and regulations about having new partners that weren't there before. The foundation of trust, respect and freedom that we start with is what I want to carry through the relationship. Yes, our obligations will increase as we get more entangled, but I don't need to set rules/guidelines/protocols/boundaries that will ensure my partner will maintain his responsibilities. That is not the type of relationship I want.