There are no rules
You could turn up as rommel if you like... *but*
Some groups ask you not to wear officer kit as it makes it confusing to work out who is leading the sides (usually the sides organisers will be badged up from the rank of Major to Sgt Major and cover the senior ranks).
If you're playing in a squad it looks really stupid if you're badged up as a captain but the organiser appointed squad leader is a CPL.
Thats the hard and fast of it. CIA have a no 'officers caps' rules for players as peaked caps are obviousl signifiers to the faction leaders.
Now less hard and fast is 'appropriate rank'. Generally try and badge up as something that looks right for your age and to a degree you have confidence/experience to get away with.
teens with a rank higher than corporal tend to look silly, same for officers, you can just about get away with 2LT but it again is pointless if you're not running a team or side.
Likewise badging up a Sgt Major is a big daft if you're 8stone when soaking wet and have a squaky voice... while its all dressing up at the end of the day we look silly enough as it is without making it easier for other living history types to mock us 
As a last rule, you tend to find that those who *really* want to play officers and senior NCOs are usually the least well suited to do so.
Personally if i'm running a side i badge up as a major/sturmbahnfuhrer, it's about right for my age, i've got the experience to get away with it (ie i've always had management jobs for the last ten years so know how to motivate people) and i've also got some military training in commanding small units.
If i'm just playing I either badge up as a private/gefreitor or a corporal/obergefreitor (usually the latter as a lot of organisers know me and ask me to run a sub section).. if i know i'm going to be running a sub group in advance I go as Sgt/Scharfuhrer.
As a young lad make the best of it, start of as a basic private and build your rank up with age and experience... most of us cant get away with the 'young green recruit' look, which is a shame as the ratio of NCOs to privates is weirdly top heavy.
In essence where what you like but you'll be helping organisers and making their events look better by badging low.
"I think we are in rats' alley - Where the dead men lost their bones."
And very good it is too.
If i could offer a suggestion.
The rank of stabsgefreitor was usually for those unsuitable or unwilling to progress to a more senior rank... usually very old of daft long serving soldiers or those known to be unlikely to fulfil an NCOs roll for very long. As far as i'm aware it just gave the old hands a little bit of seniority over young buck obergefreitors.
Again some men were given this rank as they refused promotion to full NCO.
"I think we are in rats' alley - Where the dead men lost their bones."
Yes, that seems to be the common consensus.
Either way, you're unlikely to find one in a front-line unit, so probably not worth bothering about.