I have never had a problem introducing the new 8-week-old puppy to the current residents. I have had three Brittanys in the house 3 times now. Puppy meets the current two ... in the first room off the garage. No food, toys, or rawhides. No leashes. I find a leashed dog is the most "defensive" or nervous when around unfamiliar dogs.
My daughter adopted a really nice dog a couple weeks ago. I puppy sat it twice. Last Sunday they came over. In that case because he was 5-6 months old and a bit cautious and we did one dog at a time. We started with my 8-month-old pup. Went well they played. Next was my 14-year-old female Brittany ... went fine ... he let her know she does not play rough. Last was my 8-year-old male Brit and current alpha of the dogs ... He did fine too. Within an hour the 4th seemed like he was never not here. We will be watching him this summer when they are on vacation.