Let's see...our rules are agreed upon. They are also flexible.
I am the more poly of myself and my husband. We have agreed that, because I am bisexual, I am allowed to date other women, but no men. Sex with another woman is allowed only after he has approved of her and she and I have established a true loving relationship. These things work for me since I don't desire other men, wouldn't want to be with someone my husband was uncomfortable with, and don't enjoy loveless sex.
Beyond that, the woman (and only one woman) has to want to be involved with our family. My husband and children are very important to me and I wouldn't want anyone who didn't respect that and want to build a relationship with them as by being poly I want to add to my family and not take away. This doesn't mean an intimate relationship with my husband. He won't be nor do I really want him sexually involved with my gf. But a friendship should be there. The gf is free to have another relationship if she chooses, but there has to be some form of polyfidelity. In other words, no casual sex from either of us or her and her other.
Beyond that, full honesty and full disclosure. Hubby and I are primary, at least in the beginning stages as we do have children to think about.
However, with my last gf, we were ready to change the rules. We would have moved on to a full triad if she had been willing. The rules are there for guidelines and taking first steps for everyone's comfort and safety. The good thing about them is that they can be amended to fit the individual situation at any time by discussion and agreement.
I'd rather be hated for who I am than loved for who I am not.