If you make a post reserving a regiment it means you are going to be using the name. It doesn't act as "we might use this name but maybe not, so I'm going to take it anyway and decide".
If I misinterpreted the original post, my apologies, good luck.
Sorry to say but i am afraid you wont be able to use 51st_(rank_(name)
As the 51st Imperial Guards took that tags.
So i guess 51st_York or idk.
That's not the case, imo. Regiment names are the issues, I see no problems when it comes to tags.