yes dogs are omnivores, this is why I feed plant matter as well as meat to my dogs

as for corn not being bad? no so. corn is not considered bad because it isnt meat, its considered bad because it isnt digestable. it really doesnt matter where the proteins come from if they cant break it down in the first place. this is where the problem lies in many grocery store brands, they include the protien from the high levels of corn in their break down, but the dogs cant actually GET any of the protein from the corn, making it a completly usless filler, and considerably dropping the actual levels of protein in the food.

as for protein amount? it depends on the dog, you dont want to always go low protein, you need to take into account the activity level, size, structure etc.. as all of these can have an effect.