§ 61.195 Flight instructor limitations and qualifications.
A person who holds a flight instructor certificate is subject to the following limitations:
(a) Hours of training. In any 24-consecutive-hour period, a flight instructor may not conduct more than 8 hours of flight training.
(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.
The changes to (c) were implemented sometime in 2009 from what I've just read.
This ruling (PDF link) clarified stuff and was connected to the changes.
Therefore it looks to me that it's legally now useless to hold Flight instructor instrument airplane certificate without "airplane single engine" and try to instruct and those who do so after 2009, are skating on very thin ice. True, doing it as initial may save some hassle/stress of first instructor checkride, but..