I would go to great lengths to make sure both loops are a minimum O-72. If you ever want 21 inch passenger cars.
Due to space limitations there are a few spots on the inner loop that had to be O-60, but it works enough for me. Luckily, the majority of the layout is O-72, O-84, and O-96 with the outer loop being O-72 minimum. The PRR Broadway cars in one of the pictures are the 21 inch variant from Lionel. They're a real pleasure to run