The only rule i am aware of is that my spouse told their other partner that there is not "allowed" any suggestion that Spouse and i break up. Spouse's other partner is "not allowed" to say, for example, "why not get a divorce and be with me only?" or else Spouse will... I don't know, get mad? Break up with Other Partner? Do a three strikes and you're out? I do not know what. What i DO know is that is SPOUSE's "rule"; i did not tell him or her to say that to their other partner.
We also had a "rule" that we would not allow drama from our other relationships to get into our relationship, but see the thing is... That goes all ways. That isn't really a "rule", it also applies to me and i have been the one most "guilty" of bringing drama into THEIR relationship. I also have a partner to whom i am not married and do not live with, whom i have known for twice as long as my Spouse, and we happen to agree that bullshit and drama also has no place in our relationship.
I am sure i sound hostile and judgmental, but it is a FACT that if you have a grip on yourself and know the difference between fantasy and reality, you don't need "rules" for other people. Having your own rules and codes of conduct for yourself is fine. But i believe that if you need rules in order to control what someone else does, you are completely off your game.