Inter canine tolerance
Add a puppy to an existing pack of dogs and you will see many different degrees of tolerance and intolerance. When we added Luke to our pack; Tilley was none too thrilled with the idea. She hated him for months; infact she hated him for so long that I thought I may have ruined her life forever.
It was months after he joined us that she first allowed an approach from him without trying to attack him. At that point he was very wary of her and never pushed any of her buttons. Now 8 years later she would never admit it but she loves him. Jessie on the other hand has an agenda; mount and get her message across.
For a Jack Russell she is pretty great; she tolerates as long as the newbie knows that she is the queen and she will drive that message home until it is understood. There are things you must watch for like bullying or over the top dominance. But what is toughest for many is the fact that you have to let the adults growl and display. This is their way of setting things right with the new member of the pack.
It is definitely easiest to add a puppy to your pack; they are much easier to bend and mold. And you cannot force anything; your existing dogs will take their own time to decide how and when they will accept the newcomer into the pack. A watchful eye and mediation is often called for before that moment in time when they are completely accepted.