Nobody uses 1 inch per gallon anymore, we calculate bioload per square inch of tank space along with individual species space needs. I'm surprised you find a 3" angelfish to be acceptable. How about a stunted borzoi who has suffered malnutrition and poor living conditions. Stunted fish rarely live as long as they should because their internal organs suffer in the process. Ideally, an angelfish will live past 15 years if properly kept. A goldfish certainly won't live long stunted, and they don't grow to the size of their container, that's a myth.
Mollies get up to and exceed 6 inches when they are raised properly. They grow slow though and usually die long before their potential lifespan.