Which ever rating under your CFI (Airplane single engine, multiengine, instrument , Heck Helicoper, Glider, whatever....) is your initial. Just as doing your comm airplane (fill in the blank,asel, amel, ases, ames) first is your initial check. ANYTHING after that is an add on. There is no requirement of which to do FIRST. If all you have and ever plan to fly is a twin seabee then just get your comm AMES then do the instructor airplane multi then instrument (OR) instrument then multi..... There is NO requirement to EVER do anything in any other order than YOUR choice other than you MUST get a comm. pilot cert before a CFI and the comm must be of the same cat. and class of the CFI requested. (don't only have the AMEL and try to get the CFI ASE, ect.)
I liken this to the same nonsense that certain FSDO's won't allow line captains (FAR 121 airline types) to renew the CFI based on the regular evaluation of the fo's (which is a stated part of our jobs in our FOM) yet other FSDO's do so without issure..... it's government as usual.... whatever that office manager wants is what goes, if you don't like it look to a different office.