I had an 11yo staffy x red heeler when I adopted a maltese shihtsu puppy. At first, the old boy was a bit dubious and grumpy but he got over it pretty quick. I think puppy gave him a new lease on life, he had a new pep in his step. He was eurhanisef and a few years later I adopted a staffy x kelpie. No issues whatsoever. Maltese shihtsu has always been the boss, bossing him around, which can be funny to see

I think it depends on your dogs temperament and you’d know them best. How do they normally react to dogs in passing when outside or going for walks? And with others? The best thing you could do is trial first and introduce them slowly. Im sure like any new addition, there is always an adjustment period

We had an 11yo dog when we bought home a new puppy. Everyone said the older one would react badly to a puppy at his age. They were fine, spent ages playing together. I think new pup gave old dog a new lease on life