[SlantG]

Here's the simple answer: To avoid an FAA enforcement investigation, have the instrument rating for the appropriate aircraft category on the flight instructor certificate.

Ref:
61.195(c) (an IPC is "for a rating", limitations)
61.193(f) & (g) (privileges)
61.57(c)(2)(iv) (authorized instructor)
61.1(b)(2)(i-iii) (defines authorized instructor)

61.1 (2) Authorized instructor means--
(i) A person who holds a valid ground instructor certificate issued under part 61 or part 143 of this chapter when conducting ground training in accordance with the privileges and limitations of his or her ground instructor certificate;
(ii) A person who holds a current flight instructor certificate issued under part 61 of this chapter when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate; or
(iii) A person authorized by the Administrator to provide ground training or flight training under SFAR No. 58, or part 61, 121, 135, or 142 of this chapter when conducting ground training or flight training in accordance with that authority.