I think if Yuffie was a cop, she wouldn't be from wu-tai, and would probably be from junon or something. She's too much of a brat to pull off being a cop, you would have to change the attitude to a much more mature one to help justify being a cop. I think it would've made for a bland background story, and of course there's already a gunslinger in vincent so I don't think it would have turned out well.