The hot shoe vs. PC socket thing is in my opinion, nonsense. The two will have the same or similar circuits. most likely connected in parallel. I think the warning about hotshoes is the worry that a charged flash could be pushed into the hotshoe and the centre contact could momentarily connect to a pin it's not supposed to connect to. I can't see that the actual trigger circuit will be any different to that connected to the PC socket.
I find it strange that an RZ67 is rated that low. I know the shutter is electronically timed but I would have thought that the shutter is still triggered by mechanical contacts.
I'm sure that when the RZ first came out, people just used them with their existing flashes without any worries. This sync. voltage thing is a more modern thing.