I can speak directly to this...I did it and it worked. All you need is to be a NATOPS or Assistant NATOPS instructor (I'm talkin' Navy here since that's what I know). If you were a helo IP then you'll get a CFI-Rotorcraft. If you were a T-34 or T-6 IP then you'll get CFI for single engine. If multi-engine (C-12, P-3, E-2, etc.) then you'll get the CFMEI/I. I was a rotor guy and a multi engine guy so I got the CFI/CFII for Rotorcraft and Multi Engine. Note: I'm not a CFI for single engine stuff...so I can't take out a buddy who wants me to sign him off in his little single engine piston job. Of course none of that matters because it's just resume fodder...but there you have it.
A note on the CFII...any Naval Aviator (according to the 3710) is (or was back in my day) eligible/qualified to give anybody an instrument check. Wether your squadron or training command actually did it that way is another matter and beside the point. I found the chapter and verse for that and brought that with me to the FSDO since where I flew (Station SAR and C-12's) we regularly gave each other our instrument checks wether an IP or not. It was a moot point as the gist that I got was that if you were qualified to get the military competency for CFI, you got the CFII too. I never had to produce my proof from the 3710, I think they just "get" that. Of note, the MCI (Mil Competency Instructor) test covers subject matter for the whole lot anyway.
So....I walked in with my old NATOPS ANI designation, my ME Instructor designation and MCI test score and walked out with the whole shebang.
One last note - if the FSDO you go to doesn't give you satisfaction, go to another.
I did the Sheppard Air prep for the MCI and would highly recommend it.