I use AES for the jetway movement and pushback, and I use the cateringvehicles, fuelvehicle, bagageloaders and stairs from GSX.
The only problem that you can run into, is having double stairs (from AES and GSX) when you're parked at a parkingspot that has no jetways. Because you can't disable the front exits in AES (otherwise the jetway(s) don't move), you're going to end up with stairs from AES and GSX. You can disable the stairs in GSX, but that results in not having the stairs with the groundhandler logo. But that's just a personal preference. If you don't care about the logo, you can just disable the stairs in GSX and use the stairs from AES.
Jon