I am going to answer part of my question. It appears, after some research, that STL format just has a generic unit, so I guess my answer to myself would be: send the STL file to the printer with instructions as to which scale the generic unit is (e.g. 1 mm) and his CAM app will allow him to set this scale (hopefully).

Still open to suggestions for checking the STL files, a lot of other CAD/3D packages have trouble exporting STLs because there is a redundant normal/facet tag in the file format that can be interpreted/generated loosely by various apps.

Thanks again.

Adam.