Hello, Elle!
Yes, the green berets are the cabbage hats.
It's a term used mainly by the Paras and other regiments, and it is not said in an endearing fashion!
When you were asking about "informer" I wondered if you had thought that "OR" (other ranks) was the serving men, but no, you had twigged it was "RM".
A straightforward clue, Elle, but you may have overthought it, just a wee bit!
You are going the right way, though!
Was a good puzzle, I thought!
And I liked the new words (to me!)
The Friday puzzle is usually fairly tricky.
Hope so!