There's no need for the PaintKit documentation anymore, unless you really want to "paint" a new livery.
The latest Live Update we released August 10th, added the ability in the parking customization screen to override the default priorities and specify your own operators freely (of course, their livery must be available in GSX).
You can also edit the parking Type, which might be useful to fix some 3rd party airports (FlyTampa used this method, so you might want to do this in your case) where everything is flagged as Cargo, which would cause GSX to place only cargo operators there.
We haven't update the GSX manual to better explain it but, I think is quite straightforward, and as with the other options in the parking customization dialog, you can apply the changes to a single parking, a group or parkings, a whole terminal or even the entire airport, depending on what you select from the list on the left side.