Are the fold-over infinity stops and focus scales common to all of the graflex models? In other words can the infinity stops that came off a 4x5 camera be installed on the rail of a Century 2x3, and the same with the scales?

Pacemaker Graphic to Pacemaker Graphic (that includes the Century), yes. All use the same fold-down infinity stops.

Other to Pacemaker (or Pacemaker to other), no.

I'm not sure about the scales, but remember that they're very lens-specific, i.e., for actual, not "nameplate" focal lengths. The two can and do differ.