We got Tommy when he was 6 months old and Tasha was 2, Willie (rest his soul) was 12. Willie had nothing to do with any of them, if they bothered him he gave them a growl and a lip curl and they just learned he was a grumpy old guy to leave alone. Tasha had always been wonderful around other dogs, but when she was a baby we lived in an apartment, the only one in town that allowed dogs so everyone living there had at least 1 dog, and therefore she got a lot of socialization that way, since we all took the dogs to the same area for exercise and potty.
She always seemed to accept a submissive role with dogs she met when she was younger, and thus never once had a fight erupt, but when we got Tommy, she gradually took over the role of top dog and today there is no question at all between them that she is the boss, despite his bigger size. He doesn't have an aggressive bone in his body and I think she learned after awhile that he was easy to push around :p. I agree that looking for a dog with a submissive personality would probably be in Samantha's best interest, the only problem is sometimes at first meetings things can go fine but when they realize they are now "pack mates" they look more seriously to define their roles. I do think if you get a pup that is under a year old it will more likely accept Samantha as alpha.
It also may take a week or two before you get a good idea how things are really going to be. When I first moved to MN before I got a job and so on, I lived for awhile with my mother-in-law. She had a little male sheltie named Devon and I had Willie with me (Cody was staying with my brother-in-law) Anyway when they first met, they were ok.. then a few hours later there started being friction. Willie, not normally aggressive, decided Devon couldn't live upstairs, and chased him down stairs every time he tried to come up :o. I stepped in and did not allow him to do that, he had to go into another room when he did it. In a few days, that dissolved and they became the best of friends, Willie got closer to Devon than he had ever really been with Cody! I do think there was more friction to start off with because they were both well into adulthood when they met.
You can do a few things also to show the pup that you place Samantha in a higher role too, such as feeding her first, giving her attention first, going out first, etc. I'm no expert but I think if a pup sees the rest of the "pack" treating Samantha as higher than her/himself, it will make an impression.
Tasha seemed very happy to have a playmate as Willie would not play or pay any attention to her. It is so wonderful to watch them playing together and being such good buddies :)
