While I do touch on running a campaign with Vorlon player characters (ok, it's the 21st century. What are you doing this century-long round?), the character rules there are mainly for NPCs. While running first ones through GM fiat is fine for most games, I wanted some mechanical support for games where the characters are encountering First Ones on a semi-regular basis, like a high-powered Ranger game or something.
While the First Ones are immensely powerful, they're not omnipotent. They can do _almost_ anything, but fundamentally they're working under the same rules as the rest of the universe (they just bought _all_ the expansion packs and supplements, the little munchkins). My real guiding light when doing those rules was Interludes and Examinations - I aimed to design a system which would support Kosh calling a GIANT UBERWARFLEET in to wipe out a Shadow force, but would also punish overextending yourself.