Does the 508 have a selector for choice of speeds displayed in the setup menu?
Regarding shutter speed and flash with an RB67. The RB lenses are leaf shutter, so the shutter is synced at all speeds. I think someone said that already. Therefore you can use any shutter speed for a given shot, with the same aperture (f-stop) provided the flash is on automatic, and set for the proper f-stop and film speed. If you took several shots of the same subject, varying only shutter speed, you would find nearly constant exposure on the flash-illuminated subject, and varying exposure on the background ranging from underexposed for fast shutter speeds and overexposed for slow ones.
I have an RB67 and shot it hand held with a Sunpak 622. The slowest shutter speed I can reasonably shoot without camera movement/blur is about 1/60. Maybe I can manage 1/30 if I have time to compose, catch my breath, etc. But for a wedding, party, whatever, I recommend 1/60 or 1/125 (or is it 1/100 can't remember). Set the flash on automatic and the film speed/aperture you are using (f/5.6 is good) and shoot away.
As with everything in photography, practice will allow you to develop a technique that you and your clients like.