Duke grew up around cats, I had cats and my mom had a rottweiler when I got him, but last year I rescued a rabbit so he had to adjust to that. for a few days we had some problems with him peeing in areas around the house, and he still gets jealous at times but they got used to eachother. In 5 weeks I'm getting a new Yorkie, hopefully all will go well. I have been away from my baby for 2 months now
We chose to get a girl since Duke is quite territorial, I thought it may help them get along better.
I think the best way to get them to adjust to a new one in the house is to give them the same amount of attention, so they know they aren't being replaced.