When it comes to pet adoption, there's really no magic formula. 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.