Why does an A318 need less runway than an ATR72? It makes absolutely no sense!
My suggestion is to fix the runway lengths and also allow "reduced-load" flights.
Eg. if you fly a B744 between two nearby airports with 8000ft runways it can do it, but attempting to use the same B744 between one of the 8000ft airports and one across an ocean, it won't work. This could also work if you have less pax seating (and eventually cargo capacity) on the plane.