One thing I do not understand is why you cannot have the fuel truck and catering vehicles, or for that matter fuel truck and boarding, done at the same time.
This has been discussed and answered so many times on the forum: in order to reduce the chance of vehicles clashing into each other. GSX vehicles don't have real awareness of their own relative position.
Trying to do so would put a big strain on performances, because each vehicle should check the position of all the other ones at each frame, which would result in flooding the sim with too much data, and possibly affecting all other addons.
And, even if vehicles had full awareness of each other, a whole new class of problems will surface, like traffic "deadlocks", which will be very common considering the limited apron space.
So, to simplify all these issues, we try to have some vehicles at predictable positions, which means not allowing some services to be run at the same time.