Some of the vehicles for the lack of words look default and not highly detailed as AES vehicles
Then you are probably seeing default vehicles or, perhaps, vehicles supplied with a scenery, not GSX ones.
Is there a way to eliminate the fuel truck
Just not call it. No GSX service comes automatically without you calling it.
and rear cleaning services from GSX.
There are no cleaning services in GSX at all so, there's nothing to remove, this might confirm you are looking at other service vehicles and confusing them for being from GSX, that's the only way you could say AES vehicles are more detailed.
It is planned in the future for animation to show these guys walking into the aircraft clean it and walk back out. (none of this available in GSX).
You can bet that, when we'll add cleaning vehicles to GSX, they will have superior animation, like the baggage handlers already have. The Fuel vehicles will be drastically improved in the future too.
But the main thing is can both animations support one another at the same airport at the same time ? 
If you search all forums, the most common question user pose, is how to use AES for jetways only (at those 3rd party airports that have fixed jetways) and GSX for everything else.
There are no problems from GSX side, you just don't call a specific GSX service, like the fuel vehicle. The problems are all on AES end, which starts moving vehicles automatically when you open its menu.
But for now I have to say AES is improving and with limited airports
Exactly. Just consider this: when we improve something, it affects 20.000 airports at once...