GSX system is very flexible and easy to expand.
We'll have several loaders, several passenger stairs, to suit different aircraft sizes.
Each one will have constraints (depending on its modeled size and extension) in its own sim.cfg, regarding minimum/maximum airplane door height. For example, a certain loader might say it's only suited from 2 to 4 meters doors so, if you are flying in an airplane with 3 mt door height, that vehicle can be chosen, otherwise GSX will discard it, looking for another one.
If everything is correctly set, it wouldn't happen GSX would select a vehicle that can't service the airplane, because it's too small or too big to reach for the doors (passengers or cargo)