Actually the regs are quite clear ...
That's what the Inspector thought, too. No, the regs are not quite clear. That's why there is a part 61 FAQ Document that's bigger than part 61. The part 61 FAQ is #7 on the list of FAA Regulations and Policies above Advisory Circulars and well below Public Law. Inspectors are supposed to know this. Instructors need to get familiar with it, too. Further, a FAQ exists for part 141.
One has to be really durn careful to not take away privileges of the Instrument Instructor nor add privileges to the Multiengine Instructor. An CFI-Multi can not conduct "instrument instruction" without the "Instrument Airplane" category rating. There was a proposed change in the regulation in 1997 that added multiengine instrument and single engine instrument ratings to the instructor certificate. It didn't happen.
An "Instrument Airplane" (only) Instructor can conduct training and endorsements for all of the tasks in the IFR PTS including IPCs to a non-certified pilot in any airplane category aircraft for which the instructor holds the appropriate pilot certificate. The instructor must have five hours of PIC time in make and model in the case of a multiengine airplane.
If you do get a call about your following policy, force the inspector to put the exact charges in writing, an LOI. If you correct the Inspector, there will be another charge, and the Inspector will not stop digging until he/she finds something that will stick. I wasted two months going 12 rounds with the fellow instead of forcing an LOI and taking it to Legal.
All of us are only one ticked Inspector away from being a ground-pounder.
Fly SAFE!
Jedi Nein