When you bring different ages into affect, the animals aren’t used to it and naturally will be curious and possibly reject the idea at first. But animals who are used to it from the get go will accept new animals nicely.

My family always had both dogs and cats and normally the dogs outlived the cats, but we never had a problem getting another cat and bringing him into the picture because the dog was already accustomed to getting along with other cats. This made it easier if the cat was not, because the dog was never threatening and never gave the cat a reason NOT to trust it.

As the animals get older, they will likely know the difference between a nice cat and a not nice cat. Neighbor cats were often scared of my dogs, and my dogs knew it and would chase those cats (without causing harm) yet they never tried to chase our own. They knew the difference.

And normally the same goes for cats (there are ALWAYS exceptions) but most cats won’t trust a stray dog just because their dog at home is nice. Animals know the difference.