How can I get my chihuahua puppy to get along with other puppies?

She's nine weeks old and I know chihuahuas tend to be aggressive, but I want her to play with my cousin's puppy so she won't be lonely. I know chihuahuas get along better with other chihuahuas but I don't know anyone who owns a chiuahua puppy.She has some of her shots and she has an appointment in 2 weeks to get her next shot. The vet said not to take her to dog parks, pet stores etc until she has all her shots and since there is a high in parvo in the city where I live she recommended I keep her home. She's generally good with humans, but she'll bark or growl occasionally at guests and my cousin's puppy.

You need to get her around as many other dogs as possible. Right now she is in a very important period in her life. Up to 12 weeks of age, puppies learn a lot about the world and it shapes them as adults. If she is not regularly socialized with other dogs in a healthy way, she will likely never get along with other dogs. This goes for everything else as well (grooming, nails, touching feet, large people, small people, white people, black people, loud noises, etc.)You need to take her to puppy classes and puppy socialization. They usually go together. She will be around many other dogs her own age and be in a safe and healthy environment. Make sure that she is up to date on her shots. She must be socialized with multiple dogs. If she is just socialized with your cousins dog, that is the only dog she will get along with. And she may end up not getting along with that dog later on.