Congratulations on your new puppy! There's a lot of good information on the forum. I recommend you take a look at the puppy forum and pay attention to the stickies on training expectations and socializing your puppy. I also strongly encourage you to crate train your pup. Is the yorkie crate trained? It's helpful for the dogs to have a safe place that is theirs. Another very important part of bringing a new puppy home is to find a good trainer. Especially since this is your first GSD, you'll both benefit from continued obedience classes.
Regarding the yorkie, I would be very careful allowing them to play. Your yorkie is a senior, and she may decide that a pushy landshark that will quickly out weight her isn't too much fun after all. I have a 10 month old GSD and a 7lb chi/dachshund mix. When I brought him home at 12 weeks, he was already almost 3 times her size. His version of play could easily hurt her, so my goal was to make them more or less neutral with each other. Bear was always on leash around the other dogs at first so that we could control him and ensure that he wouldn't harass them. When the puppy wants to play, redirect the puppy to a toy and play with him. You should be the fun source of play, not the yorkie. It took a lot of consistency and patience, but now they get along well. Here's my boy with my little dog and my parents' yorkie mix.
Even though they do great together when I'm with them, I never leave them together when I'm not around. With such a difference of sizes, an accident could easily hurt or even kill the smaller dog. They are both crated when we're not watching them.
The cockatiel is a little more tricky. GSDs have a lot of prey drive, and it will be a lot of work to have the two co-exist peacefully. I would be extremely cautious and always have the puppy leashed around the bird; one accident and you could have a dead bird. Hopefully someone with birds and GSDs can chime in here. I had a cockatiel growing up with 3 dogs, but they were all older and very mellow dogs. We never had any issues, but we were very careful with management.
Good luck! Please share some pics of her when she comes home.