You know, I have to admit that even the Lithosphere communtiy management team struggled to come up with the naming of our rank structure. We had a pretty good idea of how we wanted the structure to look and feel in regards to what requirements it would take for a use to attain a rank, and what permissions (role) they received once they got there, but naming them was tricky.
I like your inital thoughts with the 5-within-5 forumula, indeed some of our gaming community have had in excess of 100+ ranks, but when you're first launching you probably dont need to worry about having more than 20 or so. The reason I say this is because once the dust settles and you get a feel for what people are using the most (tags, Accepted Solutions, kudos, etc), that will help you determine what might be good things to use in ranking forumulas.
The one golden rule I would say is to make sure none of your ranks dont sound too derogatory (like n00b, toddler, or something like that), and that they at least make progressive sense (acolyte, priest, bishop, arch-bishop, pope, etc) so that those people that are jumping in will get the lay of the land immediately.
I'll warn you that it is really easy to get hung up on the naming structure of ranks (I've seen big enterprise customers spend hours across multiple teams trying to wordsmith their word structure), but IMO that time is better spent working on more tactical things that are going to make the community successful.
