my question to Umberto is not "how", but rather can we as end users have the ability in the editor to choose "which" tug we require in each parking spot.
My reply already included all the information needed: you can control the truck assignment a bit, either by playing with the airplane weight and/or with the parking spot size. And, you can control it entirely, by changing the pushback truck parameters in its own sim.cfg file (keeping in mind they might be overwritten by GSX upgrades).
You can't independently control truck type assignment on a parking by parking basis, because the assignment is dependent only from the aircraft weight and the parking size, there's no connection between the truck and a specific parking number in the airport customization.
