Hi Oliver,
Let me comment on this point:
I've tried also adding the ppd inside the PlanetPress Design but that will work only on that particular printer.
Unfortunately, it is not possible to make a "universal" paper handling solution that will cover all printers. The way printers work can vary greatly between each others, and that's why you need a specific ppd for every printer. The "Default PPD" is a generic ppd file that will work on some printers, but definitely not all, and it's always recommended to use the correct ppd for each model of printer. Otherwise, it might not work.
In the end, you may end up having to use multiple .ptk forms for your solution, and implement some logic to have PlanetPress select the correct document based on what printer you are sending the job to.
Regards,
Raphaël Lalonde Lefebvre