The FF unit will work with any rails at pretty much any spacing, as long as the rails are somewhere between 14 and 16mm, although 15mm is the design spec. With a very small part addition, 19mm will work too. Dan's mount design is incredibly flexible...the best I've seen anywhere. Both height and approach angle can be adjusted independently. It can also be mounted on either side of the rails.
Basically, it will be 100% backwards and forwards compatible with any rail systems we've shipped, and will definitely work with any 15mm rails system in existence.