Yes, we'll have to do a new kind of animation for the Fuel Hydrant, for airplanes with a lower wing.
Secondly, and more importantly, it doesn't fill the tanks.
If an airplane is flagged as using a custom fuel system, and the Maddog X surely is, GSX is *NOT* supposed to fill its tanks. The airplane is supposed to fill itself as it always were, and GSX is just waiting for that to happen, by monitoring changes in the fuel load, which can be either immediate or progressive, depending what the airplane is doing.
So, as explained on the GSX manual in the chapter that discusses refueling using an airplane flagged as having a custom fuel system, you must be sure you use whatever system the airplane has in place to set the fuel, AFTER you see the "use the aircrafts refuel system" message from GSX. That message indicates GSX is waiting for the fuel quantity to change, to consider refueling done.
This is the general rule. However, since the Maddog X has several GSX integration features coded, it would be best if you asked Leonardo some clarification about the specifics, since they might have a specific procedure to follow with GSX.