The category refers to the pin sizes, width at the lower pins, and mast height of the top link on the implement. The SAE category ratings are generally based on tractor hp at the PTO not necessarily the rpms or rotational direction.
Most earlier GTs had 2,000 RPM ptos that turned in the opposite direction as 540s, but very few of those are made anymore. The 540 rpm pto is available on most large GTs these days, including the JD 425/445/455 series. I'm not aware of any companies that still build implements or GTs with the 2,000 rpms. Even Befco and Woods make their Cat 0 stuff with 540 rpm gear boxes and shafts.
As to the poster's questions, I have several Cat 0 attachments that came with bushings or a set of Cat 1 pins and a second set larger holes for their use. It's easier to make a Cat 0 attachment work with a Cat 1 tractor than visa-versa. Just make sure the lighter duty attachment will handle the power of the larger tractor.