If you don't get a satisfactory/economically-sane answer on actually obtaining FM9 licenses (which I suspect is possible through Adobe Volume Licensing) ...
Presumably the other 2-4 writers are on newer versions of FM.
If they are on FM10, you're covered, because they can save as FM9. They need to avoid new features added at FM10.
If they are on FM11 or 12::
If the other writers are on older versions of FM, also no work-flow problem. FM9 can open their work, but those authors obviously can't use any FM9 features, which might be a document content problem, in which case upgrade them to FM12 and use the MIF route.