Bringing a new pet into a new environment can be stressful to everyone (humans, current pets, new pet, etc). It is important to give time and space to all pets in the environment - to allow them to decompress. This helps them learn about the new situation and environment without having to react. They can become more comfortable and aware of what will be expected from them. This article talks about fostering but is just as important to adopters and even those who have multiple resident dogs that aren't getting along.
How to introduce dogs to each other on a walk can be read here.
Have lots of high value treats (diced up chicken, string cheese) both dogs are getting high rewards every few seconds. Calm praise, create distance between dogs if you sense tension or hard stares are building.
If you have a dog who is reactive on walks to people or dogs here is a great video on a technique that will give the dog a job/behavior to do when you encounter a trigger (dog, person).This videois good for any dog/handler who need help being more focused on walks. When the weather is colder this is also a good thing to practice inside for mental stimulation/exercise.