All dogs are susceptible to the same forms of aggression. A Pit Bull is not naturally more aggressive.
Due to their strength, however, they can do a lot more harm than most other breeds. You want to start building their confidence and teaching them to be neutral around people and dogs. To do that, take them outside often. Every time you see something that they're afraid of, encourage them to approach it and reward them when they do. Teach them to work through their stress and overcome that obstacle.