There is no conclusive evidence as to what causes angel wing. It could be genetics give them a predisposition to the problem. Scientist have pointed out that feeding bread could cause it (which is carbohydrates), excess protein, genetics, etc.
They should get 1 cup of feed per bird per day - at least that is the amount I provide all of my birds per day with some leftover at the end. The other stuff is considered treats which should be 10% of their diet or less especially since you want your birds to forage as well.
I would offer treats in moderation - if there are leftovers - discard after about 15 min.