Newest addition to our family; need advice

We got our first Doxie yesterday, and her name is Evee. The breeder pressured me into taking her at 6 weeks. After researching, I worry that I should have fought more for her to stay longer with her mom and littermates. The breeder was feeding her adult dog food (found this out when I opened her care package). I have bought her puppy food, and I’m trying to mix it with the food the breeder gave me, so it doesn’t upset her stomach. She whines a lot when I am not holding her and trembles constantly. It’s been so long since I’ve had a puppy that I cannot remember if this is normal puppy behavior, or if it is because she was given to me so soon. I have started crate training, but it is not going great. She will sleep with the door open during the day, but last night was a struggle. I am trying to keep my expectations realistic, but I am worried since she is so young. Any advice you deem necessary would really be appreciated.