I haven't seen anything in the FARs, but:
-The CFI PTS says that the aircraft must be capable of performing all the required tasks.
-The CFI PTS says that the applicant must be able to perform all the tasks to a commercial pilot standard.
-The Commercial PTS says that a complex aircraft must be used for takeoffs and landings.
Thus, since you have to do the commercial takeoffs and landings in a complex aircraft, and the CFI applicant must be able to teach to that standard, a complex aircraft must be used for the CFI checkride.
Though there might be a shorter/simpler way of arriving at that conclusion.