You can use any stepper driver (or "step servo") driver that has a step and direction input. The manual mentions Gecko drives probably because there were (and still are?) very popular when the 126 came out, and PMDX has a couple of motherboards that are designed explicitly to work with Gecko drivers.
All power supplies used on the PMDX-126 are generated by the PMDX-126 from the A/C input power. As mentioned in the electrical specs section of the manual, the "PC" part of this signal name means that this supply shares the same ground as the "PC" connectors (i.e. the parallel port connectors). This is to distinguish that supply from those on J11 and J12, that are isolated from the "PC" ground.
