I agree with PPs, if you are getting another make sure they see the world in the same way as your existing dog, otherwise there will always be problems, same as with human relationships.
PoleDog (Great Pyr) is very suspicious of strangers and other dogs, he is aloof and generally disinterested when meeting them, although has perfect dog manners and is very civil and kind. Those are basic traits of his breed.
PolePup (Miniature Poodle cross) is also completely disinterested in other people and dogs, again so far (he`s still young) has perfect manners and is civil.
We waited for a pup showing the temperament we wanted, (way too many people breeding designer poo crosses which was not what we wanted at all), and although aloofness is a breed trait in both of his parents, we wanted to be absolutely sure we were giving the best chance of Pup and Dog getting on well.
They do, PoleDog has adored PolePup from the start and he can do absolutely no wrong in PoleDogs eyes. That is helped a lot by PoleDogs instincts to guard and protect his "flock" and its most vulnerable members, but it is lovely to see how close they are.
Had we got a pup of a more people loving breed, I highly doubt it would have gone so well, as PoleDog would have become frustrated at Pup bouncing up to strangers rather than calmly watching them. I had the bounciest labrador foster once, and whilst PoleDog was endlessly patient and kind with him, you could not have called them friends.
Most adult dogs cut puppies a lot of slack as they understand that puppies can be irritating little terrors. There are not many adult dogs that would actually harm a puppy, dogs with behavioural/trauma issues aside. That is one of the reasons I wanted a puppy, and not an adult/teenage dog.
All that said, previous PoleDogSr (RIP) and his predecessor PoleDogSrSr (RIP) were both Great Pyrs, very similar temperaments and were never friends.
They were both working dogs, although lived in the house as were also adored pets, and they worked very well as a team, and they would stand up for each other in a heartbeat, but they never close. Around the house they rarely spent time together, and were just not interested in each other.
There was never any aggression, jealousy or resource guarding, and both were extremely closely bonded with me, they were just colleagues rather than friends.