Don’t assume that everyone will just get along. You need to be proactive to make sure everyone is comfortable.
Take your current cat/dog’s sex, age, breed, and past experience with other animals into consideration. Opposite-sex introductions tend to go smoother than same-sex introductions. Also, if the pets are neutered or spayed, you will have an easier time.
If your present cat/dog is unlikely to tolerate the antics and energy of an adolescent, consider getting an older cat/dog. If the incumbent cat/dog has lots of energy for playing, a puppy/kitten or younger adult dog/cat is the way to go.