The reason that you see the recommendation to keep feeding your new puppy the same thing a breeder was feed for at least a month is because being taken from their mother, their litter mates and very probably going to a home where there is water different than what they are used to with people they didn't know is enough change to create some digestive problems--soft stools, sometimes vomiting, sometimes diarrhea. It's mainly just an attempt to limit the number of changes at the same time.
If I got a puppy from a breeder and I didn't think the food that the breeder had been feeding was particularly good, instead of changing it immediately I might add supplements (in the form of eggs, cooked meat like turkey, beef, chicken, cottage cheese or yogurt) and change it to something I thought was better slowly starting at around three weeks.
I don't think Nutronuggets is particularly good food but I have seen too may people hav problems feeding TOTW to adult dog and puppies--doesn't seem to agree with all dogs.
I started feeding dogs back when there was puppy food or dog food--I fed adult from the beginning at the recommendation of my vet (who dealt with mot of the show Great Danes in the area--he said no puppy food=--wrong formulation for puppy large and giant breeds). Much later some of the companies started manufacturing large breed puppy formulas which were much more suitable for the large and giant breeds--so I did change and the large breed formula worked reasonably well but eventually I went back to just feeding adult formulas from the get go.
And if the breeder was feeding the puppies a labeled puppy food I change my puppies to adult food (if they came to me on puppy food) at around 4-5 monhs--so none of my dogs stay on puppy food for very long.
About the DCM/grain free. The studies so far are in reference ONLY to breeds who bhave never been known as genetic tendencies toward DCM--There are breeds in which DCM is virtually unknown and there was and is a definite link between acquired DCM and grain free food (although it's still a question if it's just because of the food being grain free or if it doesn't contain some necessary mino nutritional element).
Doberman DCM is definitely at least in part genetic as are several other breeds--the recommendation to not feed grain free is mainly to avaoid foods that are not developed by real nutritionists and tested for appropriate results on living dogs and not just from computer models.
So most of the cardiologists (the ones my dos see included) recommend sticking with the bigger companies with long history of nutritionist for formulating food AND testing it in feeding trials (which means it didn't go to market6 5 minutes after the formula was developed.)
Hope this helps with your decisions.
dobebug