When LAX comes out and when I of course purchase it, do I need to uninstall the other add on or can they work together?
I'm not sure what addon you are referring to but, if you mean the one that just replaces the FSX default vehicles, it's just a graphic update so, I don't anticipate any problems.
I also wanted to ask if your GSX that comes with LAX will give me service vehicles at every default airport in the world and third party airports or do we only get the service vehicles at LAX and we have to buy an upgrade for the world?.
KLAX will not contain anything from GSX, because it's going to be released much earlier. And, even when GSX will be out, it will always be a separate download, it doesn't make any sense to include it in every scenery and add to the download size, when a single separate installer will handle all sceneries.
As already explained in the first post about GSX, its Trial version will work in full without any purchase in all FSDT sceneries. To get support for any other scenery out there, both FSX default and any 3rd party, will require purchasing GSX.