7 weeks? any brand of small bite puppy food. Then, when the puppy is around a year, you can go to reagular dog food, but it still needs to be small bite for this breed. They have a hard time chewing the big stuff.
Chihuahuas can be picky about their food because their mouths & teeth are so small. My chihuahua won't even touch dog food because even the smallest puppy bites are "too big" so I have to give her cat food (dry) which she loves. vet says its okay as long as its not making her sick and she is eating. If you're concerned about proper nutrition there are dog vitamins but you should consult a vet before getting them.