I that basically what I am asking is if there is a way to assign equipment by the atc call sign or some other airline info in the aircraft configuration file.
This has been discussed already, and of course is also explained in the PaintKit documentation which I've suggested to read.
The airline code of your airplane doesn't affect in any way the choice of the handling operator, and it would be wrong if it did: if your airline is flying in a foreign country, you can't expect seeing their own equipment at the destination airport.
Instead, it's the *parking* airline code that affects the vehicle selection, but not only, it's a SCORING system, the parking code matters only if there's a "tie" between two or more candidates that have the same score at that airport. Of course, this works best with addons sceneries, because they usually have airline codes in their AFCADs, while default sceneries don't. So, with default sceneries, GSX will do a choice based on the ICAO code.
As I've said, please read the PaintKit documentation to understand exactly how it works.
