I have an RB professional (no s or sd) with both plain backs and Pro-S backs that work fine. The pro-s backs are actually a bit nicer to use. I am pretty sure you can also use the pro-sd backs as well.
Sorry I can't give you a more definitive answer- but you CAN use Pro-S backs on the RB- and possibly the pro-sd backs also.
I use RB backs on my century graphic. Are the singer backs for the Century Graphic compatible with the RB67? I have heard of many folks using RB backs on Century graphics, but I haven't heard of anyone swapping the other way around.
While the singer/graflex backs are not as well built as the RB backs, they function well and the lever advance backs have the rollers to improve film flatness.
It works both ways, but the graflex backs don't have double exposure prevention. I used to use a 6x6 graflex back on an RB67 so I could shoot square frames without having to crop (or waste film).