Commercial dog foods are quite well balanced and do not need any additional food like cerelac or milk. Your puppy should be fine with the prescribed feed quantity of Royal Canin or Hills Science Plan. The feed portion will be given by the manufacturer on the packet or their website.
Also, do monitor your puppy's weight and compare to standard growth chart for your breed (your vet should be able to give you this). If he is growing well, then her diet might be fine and you might not have to do anything else.
If your puppy eats commercial dog food dry, then you don't even have to soak it. You can give the food dry. This serves the purpose of cleaning your dogs teeth as well. Just ensure there is enough water for your dog to drink, as these pet foods are slightly dehydrating.