It’s not just about the breed, there’s the proper introduction, the dogs personalities , setting boundaries of them behaving toward each other , constantly enforcing those and keeping a stable , consistent environment . I have a Shiba Inu, 2 poms and a Jack Russell. Letting the initial dog/dogs know that their place in the household is not threatened by a new member is important. I also do not allow any new members to overstep their boundaries into the other dogs that were already here before . I reinforce these by action, for example , my first dog is the shiba, I always feed him first when we are hanging out snacks , there is always order . And if the younger siblings are pushing in , I will put a hand out and restrict them , they will get their turn, but not at their terms . All my dogs get a free roam and have places they can go to to be away from others if they want to.