We recently picked up a stray dog and brought her home. She's a small dog about the size of a dachsund. When we attempted to introduce her to our other dog, she's a pitbull mix (extremely sweet, never bit anyone), the new dog started growling and shaking. I think she's probably been abused by her previous owners. How do I properly introduce the two dogs. How do I work with the smaller dog to being comfortable around the big dog?