Well first off, Welcome to K9Academy Abby. Thanks for joining up!
Its good to hear that you are doing lots of research on the topic. Pit pulls can definitely be very affectionate dogs, I've been around a few and many are really sweet. However its good to keep in mind that pitbulls in the past were selectively bred to be dog-dog aggressive (not so much people-dog aggressive), keeping that in mind will help better shape your program of socialization.
Its definitely a plus that the parents were mellow. What you have to do is start socializing him. The main priority on your list would be other dogs and cats too. Be careful that your Shih-tzu doesn't bully your pit, bad experiences like that can easily turn him into a fear aggressive dog.
Try finding other people in your area that would be willing to bring their dogs together for "play dates". Going to parks and asking people from a distance (to be safe) if its ok if your puppy met theirs is a good idea. Of course when dealing with other dogs make sure that your pup is up to date on vaccines. Before then, maybe you can carry him to see other dogs, but no touching. Always give him praise and treats for good behavior and make the experience with other dogs overall positive.
Same deal with cats/small animals.
Another thing that will help your pooch grow up to be a healthy and well behaved dog is lots of excersice. This is where play dates come in and a run or two a day.
You can crate train if you want, but I'm not entirely sure about leaving him there till you come back from work. If he sees that other dogs are walking around (while it may not make him aggressive) it could sure lead to some barking problems.
Leaving toys around is another thing you want to be careful of. If you notice that none of your dogs are possesive of toys then it might be ok, but you want to avoid it if they bicker over toys every now and then.
Well thats all I can think of for now. Here are two articles that might help a bit more:
SocializationGames to play with your dog