Re: Advice on Hombu dojo etiquette
There is no rule that one can only train in the Regular Classes (3rd floor dojo) from 3rd kyu. If you can handle the ukemi, then you can train in the Regular Class. If not, then the Beginner's Class is more apporpriate.
Often you stay with the same partner for the whole hour. A few of the instructors have you change partners once or even twice during the hour class. (for Doshu's class you keep the same partner for the hour)
None of the instructors have the students line up in order of rank. Everyone just lines up in rows, but no specific rank order.
Do not bring your dojo slippers, if your dojo uses them, to Hombu Dojo. Only Doshu and special guests wear slippers inside of Hombu Dojo.
You do not need a letter of introduction, though the office folks will not turn it away if you have one. Some sort of proof that you belong to an Aikikai-affiliated organization or dojo is necessary, or you will, as mentioned, have to pay the Aikikai membership fee.
Have good training!