My husband Dan and I have a new 8 week old Boxer puppy named Autumn Star. When we picked her up (1.5 weeks ago), she was eating mostly rice cereal (baby rice cereal) and a little puppy chow. We took her to our vet the next day, and he recommended decreasing the rice cereal and increasing the puppy chow (she's on Eukanuba Puppy Chow). We have slowly done that. I tried her on just puppy chow alone but she didn't seem to go for it, so I give her mostly puppy chow (3-4 times a day) plus some rice cereal in it. She's doing well on it. I'm also starting to add in some other stuff - all I've tried is a little cooked chicken at this point, just a few pieces added to her dish. She also has swiped a little adult dog food out of our other dog's dish and has done fine.
Remember, these little ones need to grow - I don't think she will get fat as long as she's getting plenty of exercise. Star loves to play and bat balls around, etc. I've been taking her outside a lot to walk/run in our yard.
Good luck with your pup! I grew up with Boxers, but this is my first one in over 20 years. They are great dogs.
Janice Armstrong