These airlines are going to have hard times once the game heats up. I have max 4 different types, one short-s2000, one medium, 737, one heavy medium a300, and one long haul 747. Find a type that is available now and stick with that. Md9s, 737s, md80s, 717 are a good start. The better ones are ones that you can buy new and second hand. This will allow you to grow quickly.