When it comes to pet adoption, there's really no magic formula. There are, however, things you can do to try and make your new pup compatible with the dog already sharing your life. Bringing a puppy home has some advantages over bringing an adult dog -- for starters, your first dog is less likely to feel threatened by the tiny ball of energy than he is by another adult entering his territory.
Choose a puppy of the same breed if you're worried about energy levels or personality. No two dogs are alike, but breeds do share some common characteristics. Want a bouncy, happy-go-lucky dog? A cocker spaniel or a retriever are good choices. If you have a dog who likes to run or chase a ball or sleep by your feet all day, chances are that's at least partly due to his breed. Get another dog of the same breed and you'll get a similar dog.
Adopt an older puppy -- 6 months or more -- if you have an older dog. Why? Because Grandpa Rufus probably wouldn't want to deal with a 2-month-old pup who walks all over him and drives him crazy. A 6-month-old will still have a lot of energy, but he's closer to being an adult and more likely to learn when to leave Rufus alone.
Find a puppy of a similar size, unless you're willing to be very attentive and separate the dogs every time you're not around. If you have a Great Dane, bringing a tiny Chihuahua puppy home could be dangerous. Not because your boy will intentionally hurt the puppy, but because he can do it by accident -- by sitting or stepping on the tiny doggie without even realizing it.
Choose a puppy of the opposite sex. Male-female relationships are a lot easier and less likely to lead to fights. Bring a boy into a house with another male dog and you might be in for a world of trouble. Make sure both dogs are neutered -- or at least the adult one is until the puppy is old enough to have surgery. No hormones flying around means less of a chance of problems arising.