I think it's clear that your gut is telling you this woman will cause problems. Now, whether that is the case or not, I think in any kind of relationship like this all parties need to comfortable with the others involved. You and BF may want to have some clear, honest communications about the ground rules, especially in regard to a veto power. Some relationships want this; others think it's too controlling, but you guys are going to have to hash this out amongst yourselves.
It sounds as if you need the veto power for this relationship to be comfortable for you. Work with BF to make this decision. If you decide Yes on the veto power, may sure it goes both ways. Don't grumble if he vetoes a guy of yours. Openness and honesty are paramount in open relationships.
I wish you guys well.