I don't see a problem with any of the low voltage units (6-volt and 9-volt). The problem with the Cannon flashes is they have an automatic 'sleep' mode and wake up when you focus the camera. Not going to happen on the RZ. I found that if I used the Cannon on another camera, I always had to manually switch the flash back on to shoot. Since the RZ has no TTL or such, you have to use the flashes in manual mode. I think voltage to the camera is a higher risk with the high-powered portable units like Quantum, Lumedyne and Norman. Even though the circuitry is supposed to limit voltage received by the camera, a safe-sych filter is a wise investment. I once had the synch contracts burned to a cinder on a Mamiya TLR when a studio rat plugged a cord in the wrong place and sent 120 volts to the camera. Not too expenseive there - but a major financial disaster on electonic cameras like the RZ. An EX+ replacement from KEH is cheaper than a repair.
To answer the question - yes it will work. But unless your unit (newer than mine) has a way to disable the automatic sleep mode - it is a pain. The RZ does have a hot shoe on the side - gets in the way of a bracket though. Cannon makes a 'hot shoe relocator' cord (not sure of its correct name) while it has the circuitry to manage the Cannon auto functions, it actually does work on the hot shoe of other cameras if all you need is synch trigger. I have used it with Metz low-voltage units on the Mamiya. Strictly manual mode - but if you are shooting MF you shouldn't expect the camera to do your thinking for you. (The 645 Pro-TL and 645AF support TTL but it doesn't come cheep - proprietary interfaces that cost as much as a flash unit!)
Bob