Actually they're not really ranks at all. Expert Infantry, Lancer, and MEPD Police Officer are not ranks. They are detachment specific programs that encourage and recognize costuming excellence.
On BSN "Pathfinder" just means a full detachment member, e.g. a 501st TB. Here we just use "Stormtrooper" to denote a full detachment member (501st TK).
Andrew - I'm guessing via your avatar that your a "Pathfinder" on BSN, which means you'd be a "501st Member" here. Essentially FISD, CTN, BSN, MEPD follow the same format:
Member (non-501st)
501st Member (501st, but does not have the costume for that detachment)
[Detachment Member], which is custom to that det's forum.