What’s the Rush?
As the saying goes “You don’t get a second chance to make a first impression”. The same applies when introducing a new dog/puppy to your current dog(s). My best advice? Go slowly through the process. And it IS a process. Let’s face it, it could go either way if you just put them together to see what happens. Which is what most people do. It could turn out just fine in the short term, but in the long run it could be building towards a fight.
Taking it slowly means, for the first few days to a week just allowing the dogs to exist around each other. Not interacting and certainly not playing. Have a separate crate/area where, at least the new dog is confined for sleeping, meals and down time (preferably for several weeks) and when you need a break from supervising/working your dogs. Walk the dogs together (preferably you and another person each with a dog). Having the dogs hold a command such as place or down around one another. Having them on leashes that are tied to doors so that they can’t make contact. Working each dog through obedience commands while the other looks on.