I hold commercial, instrument, multi-engine airplane certs and I am a helicopter flight instructor. Am I allowed to give Flight Reviews to fixed wing pilots as long as they are PIC?
No. You are not permitted to give training of any type in an airplane without an Airplane rating on your CFI ticket. With only a Rotorcraft-Helicopter rating on your CFI, you are only allowed to give training in helicopters. See 61.195(b)(1):
(b) Aircraft ratings. Except as provided in paragraph (c) of this section, a flight instructor may not conduct flight training in any aircraft unless the flight instructor:
(1) Holds a flight instructor certificate with the applicable category and class rating;
(2) Holds a pilot certificate with the applicable category and class rating; and
(3) Meets the requirements of paragraph (e) of this section, if applicable.
And yes, flight reviews are considered “training.” See 61.56(a):
§ 61.56 Flight review.
I figured that was the case but why can I get a flight review in an airplane and it is also good for the helicopter?
Because 61.56(c )(1) says you can. Why did the FAA write it that way? 🤷♂️