Wow, a thread started by me, no less. And here I was about to reply with almost exactly what I said in that original post more than 2 years ago.
I've since found an extremely detailed site (even if it does look a little dodgy and has advertisements popping up all the time) that deals with that, in addition to just about every other aspect of Roman military organisation, in exhausting and very impressive detail, complete with diagrams, and some very sensible analysis and references (in my opinion).
The table of contents can be found here http://www.garyb.0catch.com/site_map.html.
Bookmarks