Rather than worrying about whether the new metamour calls herself a Domme or a Mistress (I don't think it implies much), or her age/size (um?), it seems like the pertinent things here are the insecurity, bitchiness, and history of bad choices. Bad traits in any metamour and traits that in combination I don't think I would find acceptable in any person who wanted to be in my intimate life, much less someone who wanted to be dominant with me.
Did your former metamour hide these aspects of herself well, or was your ex just a poor judge of character? The tough thing about your current situation is that without knowing the new metamour or even being willing to hear anything about her, there's no way for you to begin to guess if she's a similar or a radically different person.
Also, having to hear about her existence fleetingly in casual conversation seems like kind of a harsh thing to get mad at your bf for? And now he's going to have to monitor himself to censor out any possible reference to her to avoid another fight, which seems stressful. What happened to you sucks, but I wonder about the potential to create a self-fulfilling prophecy here by making him feel so uncomfortable about this that it negatively impacts his relationship with you without your metamour ever doing or saying a thing on the topic.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.