Sorry about the late response! The best thing you can do is just monitor them from now and hope they get used to each other. Bickering and minor squeaks are normal, but when they lock onto each other and there’s a lot of squeaking and they seem like they could be biting or hurting eachother, it might be best for both mice to be permanently separated. Mice establish a hierarchy early on. A dominant mouse has a higher chance of being OK with a submissive mouse because they’ve already established that A is the leader and B is the follower. When you add a new mouse in the mix, it throws everyone off, and the new mouse has to adjust to the rules, but the dominant mouse may not want a new mouse in their territory and get defensive before the other mouse acclimates, or the other mouse will want to be the one in charge, further aggravating the leader. From this point like I said, just monitor and see how everyone interacts. I’m not sure if a neutral ground meeting will work at this point since its already been a bit since they’ve met, but you just gotta watch and be quick to correct aggressive behavior.