
What ytopu have to decide is the difference between playful vocalisation, as a lot of pups make growly noises when they play. Also if your reprimands are too harsh for a sensitive pup it may growl defensively, but still show the other signs of submission.
What your older dog is doing is perfectly normal, and if it is effective (stops the pup being a pain, and gets it to show respectful submission) then take a leaf from the older dogs book, and observe. this will help you get the discipline thing right, as obviously a dog will undewrstand the other dogs signals best.
You must be firm and consistent. When you give a command you must be convinced that you will be obeyed. this conveys authority and confidence which the pups instincts will be designed to respond to by accepting you as above it in the scheme of things. Unfortunately not being conversant with dog language we often send our dogs very conflicting messages.