Remember that current is additive, so 12 loads each of 1 amp imply that there is 12 amps flowing in the ground connection at the point they all come together (assuming a single 36V power supply). That means that point has to have large gauge wire and high current connectors. It can also cause ground offset problems for the logic, but that is what your optoisolators are for, to isolate the logic ground from the high current ground (which it sounds like will be your mother board which may or may not be able to handle it). The servos look to be capable of wanting another 1.5A each at stall so that needs to be figured in as well.
Peter