Good advice from Anya about always feeding, giving treats, stroking etc i.e. any positive attention to the older dog first. Alea is right, dogs are different and a hierarchy is important to them.
I've usually had two and often three dogs for years now. My experience has consistently been that the oldest (top) dog is the one who imposes rules on the interloper. My old rescue boy and our 2nd dog shared a big dog bed but when the puppy arrived, the gentle old dog would not allow the pup on the bed for about two or three weeks. He'd show his teeth and growl until the pup slunk off into his own bed. He wouldn't play with the puppy, growling if the pup persisted. Once the puppy was showing some respect, the old dog allowed him to snuggle up on the big bed.
The same pattern has happened with every pup (including my daughter's visiting pup) and our current senior dog. She's a big, soft lovely girl but she sets a standard, pups not allowed to misbehave. She'll bark in their faces if they get over excited and they lie down, submissively. Most dogs would never hurt a puppy but they will teach manners. I wouldn't worry but of course, keep an eye on them. One thing I'd forgotten when my last puppy joined us was how tired they get. They're like toddlers and need to sleep - I crate trained mine and when he got over tired, running around, pestering I'd pop him in his bed and he'd sleep for a couple of hours, emerging a calmer, more civilised pup. It's hard work isn't it but worth the effort and time.
