So, you want to get a puppy but you already have an adult dog - GREAT! Here's some hints that may help with your introduction.
When you first bring puppy home, try to have someone with you to help out and choose a room that has a washable surface. Allow the puppy to sniff the area. These new smells will tell him there is another dog(s) that already lives here and may help to calm him somewhat.
Time is an important factor to consider when bringing in a new puppy. You may want to plan ahead and make the introduction on a long weekend or when you have some extra vacation time coming. This will also make the puppy's transition into your home easier by starting him off on a schedule.
Let the resident dog(s) in, but don't let them rush to the pup as this may frighten him. In the case of more than one dog, let them in one at a time. They need to sniff - this is very important. If your puppy is very submissive, he may roll over exposing his belly and he may urinate. DO NOT chastise the pup, this is normal behavior. He's telling the older dog, "I know you're the boss". Once the initial greetings have been made, allow everyone to relax. Pay attention to the older dog, and not too much to the pup for the next few days. Don't expect all dogs to readily accept any new critters immediately, remember they also have feelings and may be thinking "How dare you, I'm all you need".
One thing to remember is a puppy wants to have a K-9 friend, but because puppies play very hard and can be very fresh, you must protect your older dog from being "overwhelmed". Know your dog. You may want to feed puppy in his crate (a good reason to purchase one!). Allow each dog to have time away from the other. While they are together and playing, if the older dog gives the puppy that "look" along with a little lip curl and snaps or growls - don't be too quick to step in - he's only reminding puppy to "mind your manners".
Make sure to spend extra time with your adult dog. Take him with you more often on errands, talk to him more, give him longer walks, etc. This special time will pay off in the long run.
Some dogs may take a little longer to accept the newcomer, but if you've done your homework (training) properly, your adult dog will soon be happy to accept a new puppy. You must be aware that, like people, not everyone gets along, personalities are different. After sufficient time has gone by, if this is the case, please remember that the breeder may know of a family where the puppy will fit in perfectly. Maybe the timing is just not right.
Most Newfoundland are more than willing to accept "one of their own". We have a very special breed and to see puppy and adult curled up and sleeping side-by-side is a very heart-warming feeling. So, get your camera ready for those special times!! They may only happen once and puppies grow so fast you don't want to miss a single thing!! Good luck and happy intro's!