I have no knowledge of a Team Rocket Guild with custom sprites of which you mention. Unless this was something from the very very early days of PWO, before the existing Guild system was developed, no Guild has ever had its own custom spriteset implemented into the game. (This having been said, I have often wondered why those specific sheets exist..)
Sprites from sets 3-7 are not publicly available for players to use; and exist primarily for testing purposes only. While some players have sprites from one or more of these sets as a result of rogue behaviour by a former developer, they are not officially supported and may be removed in the future (as there's plans to allow for a greater variety of sprites in a future release, with various different ideas being considered such as allowing people to choose a different "generational" sprite). Players who have these sprites may, for now, continue to use them.
(Of note, I'm also one of the Staff members using Set 5 as my in-game sprite; though I have customised my client so that this gives me a custom Gengar sprite. The "Team Rocket" sprite the rest of you see is but an illusion. I would release the files to allow you all to see myself in my "true" guise, but I fear some of you would attempt to steal the look for yourself.)
As relates to the suggestion itself - I must admit, I find the idea of a quest that would allow you to masquerade as a "villain" quite an intriguing proposal - and there is already a kind of precedent, as HeartGold and SoulSilver had a storyline which allowed you to dress as a Neo Team Rocket member briefly to gain access to a building they were controlling. This having been said, it has long been the view that Team Rocket should not make any appearance in PWO since it's theoretically set 10 years in the future, long after the team disbanded. Perhaps we could create something more original instead...