Congratulations on your new baby, around Christmas
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Hope the breeder keeps the pup until 12 weeks or older by which time she will have learned a lot from her mom and siblings. Please check out the health guarantee and get your vet to examine the pup as soon as you get her.
As for the food, I would keep her on the same food she is on now until she is really settled in before changing food. The babies have so much to cope with leaving their momma and being in a new environment.....one thing you can keep the same is the food.
I can't comment on the Parti situation, I am sure some breeder will help you on that question.
I hope you will consider getting pet health insurance ASAP as if they do get sick in the future it can get expensive. It is good to get it before they develop any issues that would be considered pre existing.
I agree that a collar can be bad for their tracheas and you need to get a harness that protects the throat. You could consider the Buddy Belt or a softer mesh type harness. I suggest you don't pay too much as your little one will soon grow out of it and you will need a new one.