Here's what you asked, in your first post, after talking about the linear promotion scheme:
"Now, here's where it gets interesting. Let's figure out how to go about actually applying real military rank and Pay Grade structure into the game."
Here's one answer:
"I wouldn't say it's very very wrong, just in need of a minor adjustment, if it bothers you. Throw in Warrant Officer and a few other Sergeant grades, Corporal, whatever."
Another: "Did you try Twilight 2000? Hey, Tramp, have you ever seen/played Twilight 2000? It was pretty good, iirc. You could maybe import it straight in?"
"You mean the games rank system?
Again it REALLY depends on the nation, and sometime the service you're modeling. In the US Army a Platoon Sergeant is an E7, in the USMC an E6. Same responsibilities, and the USMC E6 actually commands more people (since USMC platoons are larger)."
And this:
"Not entirely true (at least in the US military).
At the highest ranks in each category (E9, W4, and O8ish+) there is usually an extra Pay Grade for special cases.
Example, the Command Sgt Major of the entire US Army is technically an E9 just like the Sgt Major of any battalion, but they're at a special Pay Grade and sort of treated as an E10 unofficially.
Also within most Pay Grades there are "Time in Service" modifications. A major with 20 years in service is paid more then one with 10.
And as I pointed out earlier the actual responsibility any given rank wields may vary from service branch to branch and wildly from nation to nation. This was particularity true under the Soviet system where officers frequently performed duties that in the west were performed by NCOs."
So, those are some of the answers.