Fangita suggested the ranks back in February
in this post, but only named 5 levels. As Midshipman has no medal, this means that there was never a missing rank, it was just an (unconscious?) decision of the COs that the top RR level should equate to the ARRSE symbol for top rank. I understand Bad_CO is looking at the system.
As for symbols, I agree with the donation being marked by a chest full of gongs, as it means that the poster has paid towards the upkeep of the site (currently subsidised by the users and owners of ARRSE). However, maybe that symbol should expire after a few months (3 months?) and revert to the number of posts-based ranking.