You could use max gross weight and/or number of seats > 6. Not a lot of flight training or hobbyist flying going on in Beechcraft 18s or Cessna 402s. And people

You could use max gross weight and/or number of seats > 6. Not a lot of flight training or hobbyist flying going on in Beechcraft 18s or Cessna 402s. And people who like private jets aren't going to step down to a 6 seater.