Sounds like "Guardian" by BlackDragon6
It's something of a crack fic and has a spin-off called "Millennium"
https://www.fanfiction.net/s/720841/1/GuardianGuardian reviews
Ranma leaves Nerima and joins a special police force. Special in that it's made up of psychos, fools, and nymphomaniacs all charged with protecting Tokyo from the supernatural and terrorist forces that plague its streets.
Ranma - Rated: M - English - Adventure/Humor - Chapters: 29 - Words: 455,231 - Reviews: 804 - Favs: 965 - Follows: 535 - Updated: May 4, 2012 - Published: Apr 14, 2002
spin-off
https://www.fanfiction.net/s/2449022/1/Millennium