Question For You FAR Gurus  

Not sure I follow where you're going with the question, however.....

I have an instrument instructor certificate for helicopters, but I do not have a flight instructor certificate for helicopters.

I can teach in flight for an instrument rating, however I must be current & qualified in the helicopter we are using because I am acting as PIC while the student is under the hood.  I can give ground instruction without being current or qualified in the particular helicopter.

My instrument instructor certificate for helicopters would not allow me to teach in flight with fixed wing aircraft. That would require a FW certificate.  I'm not sure if I could give ground lessons for a FW instrument rating, but I suspect not.