For telling teachers if you are a teacher as well... I would say it doesn't matter. Every teacher is also a student. If they know you, then they probably already know you are a teacher. If they don't know you, telling them you are a teacher may make them feel a bit uncomfortable - like they will be judged.
I've not yet had any of my teachers in my classes, but that's probably because I'm a new teacher. Although, I did have one of my teachers tell me to let her know when I would be comfortable having her attend a class as she wanted to come, but she didn't want to make me feel extra nervous as I started out.
I'll limit my answers to practicing in a class rather than home:
1. What are the rules of conduct?
Maybe just some simple etiquette is try not to step on anyone's mats, don't slap your mat down loudly on the floor, don't have side conversations during class, and turn off cell phones.
2. What do you wear?
Is there any meaning to the clothes? Workout clothes because one can get sweaty. More form fitting clothes actually help teachers see student bodies for more alignment - e.g. shoulder blades collapsing or legs not straight.
3. Is there a pecking order and what does it mean?
Not that I know of - maybe among teachers, there can be different levels probably based on hours trained.
4. Where should you stand, etc.?
I think it's easier for beginners to stand in the middle length wise and at least in the 2nd row so there is always someone to follow from the front and sides.
5. Are there any rites of passage?
Not really, except there are YTT (yoga teacher training) programs. I do see some people aim for milestones in poses, such as getting heels to touch in downward dog or laying hands flat in forward fold, so reaching these could be considered rites of passage for them.
6. Is there a special 'language' of the sub-culture?
Sanskrit, for the reasons robert already mentioned. Chanting "Om" as the "sound of the universe". There are general verbal cues that can sound cheesy out of the context of yoga, but they're helpful for visualizing, such as "get more comfortable with the uncomfortable", 'listen to your body", and "observe, don't judge".
7. What does yoga mean to you?