I got my first Sheltie, Meggie, at 15 weeks and adopted Cooper when he was about 14 months old. And, he came from Judy's house.
I think all dogs are different because Cooper was not the least bid shy, from minute one. It's nice when there is another dog to show them the ropes. We did have a couple of house training issues, but eventually I learned that his signal to go out was NOTHING like Meggie's. It was definitely Mom who didn't know what she was doing, not Cooper.
Introduce them in a controlled environment and let them get to know one another while you are around and attentive. Just to make sure there are no issues. When we went to meet Cooper, Meggie sat at my feet and Cooper put his head in my lap. They instantly got along. There have been moments of slight jealousy on Meggie's part, but they are brief and few and far between.
A friend who recently adopted a shelter dog had more issues with her existing dog than the one coming into the home. So pay attention to everyone!