Rank insignia would be dependent on what you intended to use it for. If it's just for looks, no problem do what you want. If it's part of a team that uses ranks you will be assigned a rank most likely.
Me, my uniform is based on my experience in the Reserves. I was a Corporal...hence the insignia, I killed a tank up close during an excersise...so I get a kill badge. That sorrta a thing, it's a game and if the intent is fun then it's all good.
I don't think it's unreasonable to expect people to be open minded. As a German themed uniform you have to expect there will be some uneasy looks from time to time. Just be prepared to have the answers...could make for some good conversatons.
Hope that ehlps
JW