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.