I have 2 suggestions to fix 2 police roleplay issues:
First, you should change the police Lieutenant to Chief because it spoils police roleplay when you see a person usually rping a Lieutenant as a normal officer (or now an Sgt). Besides that, in real life there are many police ranks and many Lieutenants, but only one Chief of Police.
Second thing - I suggest not limiting Sergeants to 2 because it won't be fun for players who are whitelisted to either RP an average not whitelisted cop or wait until a slot is free. And of course it won't be realistic to join the police force and find out that all Sgt slots are taken. I'm suggesting making Sergeants permanent (if they're not RPing Chief of Police).
Possibly after that, there could be permanent Lieutenants just like Sergeants, but with more privileges.