For the Nikon questions:
Depends on the particular Nikon flash. TTL will (obviously) not work. Non-TTL auto modes (i.e., thyristor) will work. Manual mode will work. Some current Nikon flashes don't have an easy way to set manual mode and/or no way to vary the power on the flash itself; better than nothing, but not particularly useful.
You shouldn't need an adapter. At most, you should tape over the extra pins on the flash shoe to avoid any inadvertent connections/shorts – the centre pin and the rails are all that's used.
I don't know of any canonical references for trigger voltages, but I would assume anything that is safe to use with an F3 would be safe with the ZI, and almost certainly any AF-compatible flash will trigger well within the safe range. Which, incidentally, I would have guessed at 12V or maybe 24V (which is the oft-ignored ISO standard), so the 200+ is pretty good news to get direct from Zeiss.