What about any infection you could take back to the mum and other pups? Unless you first dog never goes out or mixes with any other dogs I would not let you take one of my puppies and bring it back after being with another dog.
Once it is 8 weeks and you decide to take it home that is up to you, but unless you are going to disinfect everything and change your clothing then you risk bringing disease or infection to the litter.
Your dog will soon get used to the new puppy provided you do the introductions in the right way. Make a lot of fuss of your original dog, give the puppy a cage or somewhere it can go to get away from the older dog, and don't let the puppy pester the older one, or if the older one wants to play, don't let it tire the puppy out. Make sure that feeding time is supervised, or one of them may end up eating all the food, and if the puppy tries to take the older one's food, it may snap at the puppy. Make sure you spend some time alone with the older dog so it doesn't feel left out.