From a comment I made in another thread about the lesson I learned about metamours:
I am generally friends with my metamours and some of them are closer to me than our mutual partner. 2 of my closest friends are metafores (a metamour from before) where that metamour relationship lasted longer and is closer than the mutual partner who brought us together.
All that said, if I have a metamour who is "a drama starter", that is not a problem between her and me, that is a problem between my partner and me because he would think that it's acceptable to be involved with someone like that.
All relationships bring conflict. I have conflicted with every metamour I've ever had at one time or another. Occasionally the personality conflict is big enough that we choose to merely coexist. The rest of the time, the conflict is like any other - we work it out and get through it.
Think of metamour relationships like in-laws. You don't have a choice who your in-laws are - they come with your partner. If your partner keeps a relationship with them, that's because they see value in those relationships even if you don't have the same value system. You can try to befriend them or you can largely ignore them, whatever you think is appropriate for in-law relationships, but they *
will* affect your romantic relationship one way or another depending on how close your partner is to them.
And if you have a problem with your in-laws, then you really have a problem with that partner for choosing to remain connected to them. If the problem is not about how they're influencing your relationship but just about personality differences, then you work through it with them directly until you find a balance you can both live with.
Poly people like to think we're inventing the wheel, that no one has ever done anything like what we do before. But most of the skills necessary to navigate poly relationships are available to us through our other relationships and our other practices.
Metamours are basically in-laws. You can't make your partner choose your in-laws based on your preferences without overriding agency and utilizing coercion so you learn to deal or you recognize that the problem is between you and your partner for having incompatible relationship goals.

"
Of course I’ll hurt you. Of course you’ll hurt me. Of course we will hurt each other. But this is the very condition of existence. To become spring, means accepting the risk of winter. To become presence, means accepting the risk of absence." ~ Antoine de Saint-Exupéry, The Little Prince