About 20 years ago, many clinical studies proved protein to be a non factor in large and giant breed puppy growth. So I would not worry about that. As for feeding an adult maintenance food, that is an old old tired theory that many breeders, dog clubs and vets still maintain, however it is absolutely incorrect. Also you have to understand that the dog food industry has vastly improved their LBP formulas. Most are now in compliance for the recommended amount of calcium and are much more safe to use than an adult maintenance formula which will have calcium levels that exceed those needed for a growing LBP.
The other huge thing with these giant breeds is keeping them at a Body Condition Score of 4/9 throughout growth (and really their whole life). Do not over feed, do not free feed. You should be fine.
