Don’t just bring puppy home!
Firstly, get the dogs use to the smeller of each other, so bring a blanket home when you next see puppy with puppy’s scent on it and visa versea.
Secondly, walk the dogs together. I know puppy can’t do too much or as much as your border terrier but they can do a little bit. I cannot stress how important it is to walk the two together and then go home. It allows the dogs to meet on neutral ground, so the original dog doesn’t feel the need to ‘defend the homestead’ against the new puppy.
Thirdly, pick up all toys. Your original dog may well not want puppy to play with ‘his’ toys, so may cause an issue if puppy goes to play with one. I have known it where having toys out has actually helped provide a distraction for the two new dogs but for the majority of situations I think it would be best to remove the bits that may cause friction.
Fourthly, do not feed them together, always feed dogs separately. Dogs can get on fine, until you introduce a high value resource (food or a faveourite toy) and all hell breaks loose. I never feed any of mine together because I don’t see the need to cause a problem.
Lastly, I would encourage all the family to have very little to do with the puppy beyond what is necessary for the first day or so. It gives the puppy time to get used to it’s new surroundings and ensures that the original dog still has the same amount of attention. When a new puppy comes a long, of course, everybody wants to play and cuddle it but this may make the original dog feel jealous or sidelined which could cause them to react to puppy negatively.
I may also consider using crate training for a puppy, that way they don’t bother the original dog during the night and you can supervise their time together to ensure it is positive.
That’s all I can think of right now. If you’re looking for a food I can wholeheartedly recommend Millie’s Wolfheart. It’s wonderful stuff. Failing that I would feed orijin or wolf tucker if you fancy feeding raw.
Congratulations on your puppy!