We are getting our newly adopted SBT today and he's six months old. My questions is - Do I need to get a puppy formula for him, or will he be okay on my 2 yr old's adult food; Natural Balance Ultra Premium?
I would absolutely switch him over to the adult food. My dogs never get puppy food. My GSDs came to me (at 8 weeks) eating NB Ultra. You might want to buy a small bag of the food he's eating now and take about 4-5 days to switch him over to the new food to avoid any sudden changes.
I would keep him on the same food he is currently eating for at least a few weeks to a month, until he has transitioned comfortably to your surroundings. Then I would put him on whatever you are feeding your older dog, gradually.