I am so sorry. Seems like I can't stop complaining about something or other. I am so annoying. Anyway. Soldiers refer to the senior ranks by ranks. E.g., Sergeant Shidehara will be Shidehara-gunsou. Senpai (not sempai) is for students and people referring to seniors. Or can also be referring to people they respect a lot.

If I am proved wrong, I apologise.

Wow. I do hope you continue. This sounds very interesting so far.