Chicken meal seems to be the best protein source for being least allergenic.
Actually, chicken is the number one food allergy for dogs.
I do not use 'puppy' brand either. Mine did just fine on 'adult' food. Too much protein in young dogs can cause too fast growth or pano or possibly OCD issues.
No, it's calcium and too many calories that cause problems, not protein. Puppy food is just fine when fed in moderation (keep the pup thin). I personally feed an "all life stages" food because I want everyone on the same formula (I buy in bulk). You can absolutely make a pup fat and increase the odds of HD, OCD, etc by feeding an adult formula.
Stay clear of corn and any gluten ingredients; they are cheaper for the manufacturer but not the best for dogs' indigestion.
Corn is not evil, but for some dogs it's a bad choice (if they are allergic, for example). Sure, crappy foods use it as a cheap source of calories and protein, but there are some excellent foods out there that use it as a carb source (which some working dogs need). Eg: Eagle Power Pack, Red Paw.
I do agree that there is NO reason to add in corn gluten. That is added to artificially elevate the guaranteed crude protein on the analysis.