Specialist diets are generally more expensive than ‘standard’ dog foods.
Those designed to keep down your pet’s weight can mostly be avoided by being careful with their diet in the first place and maintaining a strict exercise routine. Therapeutic diets are normally sold via vet practices, but are increasingly available on line. They are formulated to help with specific health problems such as diabetes and renal failure.
Puppy milk is commercially available and contains extra calcium.
Cow’s milk should be avoided all together as this can cause stomach upsets. More importantly, fresh water must always be available to avoid dehydration.
Supplements are subject to much debate. Manufacturers of many prepared foods would argue that their scientifically balanced formulations do not require additional supplements. It is probably safest to assume that if your dog is enjoying a healthy diet there is no need for anything else. If in doubt, check with your vet.