When our little Morkie was 13, we brought a puppy into the house, small like her, a MaltiPom. The Morkie lived three more years. The only thing that was good about it was the fact we had another furbaby when we lost her.
She never made up with the puppy, was jealous, angry at times, if we played with the puppy, she 'stole' the toy. After awhile the puppy even got angry that she was stealing her show. At night, the spot beside my DH on the recliner was the Morkie's. And if the puppy tried to jump off, well queen of the hill, chased her off.
When she died, I showed the now three year old, so she could say goodbye. Although for a couple of weeks, she did act as if she was hunting for the other furbaby. So for almost four years now, she has been the Queen on the hill.
We will never bring in another 'friend' until we no longer have this one. I feel if they are raised together, they will always get along. If you bring in a newcomer, there is always a jealousy and hurt feelings, especially for the older dog. If you have three or four dogs, that too may been a different story.
We still miss that little Morkie so much even tho this one is sweet as can be. But I feel we put a lot of stress in the older one's life by trying to bring in a friend for her.
That's just my story, it may have worked in other homes.