For the cops, i'd do stormtroopers, iron disipline, mechanized, sharpshooters, and spec weapon squads. Or you could do the exact same thing I just said, but instead of stormies, have carapace armour for the whole army.
For the gangs I'd give you what nightblade says, but for the extra slot I'd give light infantry, since there used to sneaking into things and sneaking back out.