I've never had just one dog & they all get along fine together.
At the moment I have three Yorkies plus a few Skyes & Cairns, which I've always bred & shown, so they're used to puppies coming along. I've never, ever had an older dog turn against a puppy, they usually try to adopt them & always play with them.
The most likely cause of problems is having two entire dogs together, especially if they've been used at stud. Occassionally two females will develop a severe dislike of each other, but more often they are great friends.
Neutered ones, especially one of each, are rarely any problem. |