I hear you Bluebird. I think your style of poly is similar to mine, in that you don't just date people, you tend to live with them. In those kinds of close quarters, it matters that everyone be on friendly terms with each other. This goes for friends of friends too. You can't have a friendship with someone whose other friend is driving a wedge between you and the someone. Snowbunny is very adamant that anyone we add to our V will have to be more than just a friend. They'll have to share a profound closeness with us, even those of us they aren't romantic partners with.
And I didn't take the opportunity to say I'm sorry that you lost your ex. From my viewpoint, things with him often seemed to be headed in a bad direction, but I didn't want to say anything because I hoped it would all work out. I am thinking your ex and his best friend both played a hand in the eventual breakup between him and you. So again I am sorry, and I don't think you were to be faulted for this one. That's my opinion anyway.
I hope it's okay for me to say these things here. It's long overdue but, better late than never.
When you have marriagelike poly relationships, you do have to be extra cautious about adding someone new.