Here's 61.195 b and c ...
(b) Aircraft Ratings. A flight instructor may not conduct flight training in any aircraft for which the flight instructor does not hold:
(1) A pilot certificate and flight instructor certificate with the applicable category and class rating; and
(2) If appropriate, a type rating.
(c) Instrument Rating. A flight instructor who provides instrument training for the issuance of an instrument rating, a type rating not limited to VFR, or the instrument training required for commercial pilot and airline transport pilot certificates must hold an instrument rating on his or her pilot certificate and flight instructor certificate that is appropriate to the category and class of aircraft used for the training provided.
Some of this verbiage was added three or so years ago, IIRC. It's still clear as mud (there is no "class" instrument rating - for airplanes anyway), but there's less wiggle room for "same category". B prohibits any flight training "off class", but then C mentions instrument training - which
might be differentiated from flight training. I'll bet the FAA's ruling made B limiting, but like I said I haven't seen it. Got a link, anyone?