Every vehicle has some assignment constraints you can tweak quite easily, usually related the airplane doors height, they are all in the vehicle sim.cfg file, located in Simobjects\Misc\FSDT*.
The [Couatl] section contains everything related to that vehicle that is specific to GSX. For example, the FSDT_Staircase folder contains this in its sim.cfg
condition = %(exitAlt)f >= 2.066
This means that specific stair (this is the covered model similar to the one in your 2nd screenshot) wil only appear if your airplane door is higher that 2.066 meters from ground.
By playing with this parameter, it's possible to control stairs assignment, and it's possible to do more complex expressions like:
condition = %(exitAlt)f >= 2.066 and %(exitAlt)f <= 3.00
Note that you might want to do this on your own custom repaints, because the sim.cfg in the original folders are likely to be overwritten by our future installers. With the PaintKit, new "USER" vehicles folders will be added, so that you can play with them freely, without worrying about GSX eventually overwriting them with future releases.