Follow your gut. You’re totally right. Never feed Senior dog food to a young dog. Your dog will be missing certain nutrients. You know even Senior dog food shouldn’t even be sold. Total sales gimmick. Puppy food is different cause it has more fats, different supplements etc. Even better, feed raw. As if in the wild they have a choice of senior food. They eat whatever they kill. Shiny coat is basically oils. You want your dog to have a shiny coat, add extra virgin olive oil or even a teaspoon of butter(real butter) to their meals. I use EVOO. I also feed them as a noon snack wild caught fish like sardines, mackerel or tripe the green smelly one that comes frozen. It’s like human hair, eat the right oils n fat you’ll have shiny hair, no flakes n no dry skin.