That wouldn't work unfortunately because members of PD/SD are not all the same. You have people that RP a detective, others RP SWAT, others RP pilots, etc. Each and everyone relys on someone else's RP to make progress for their roleplay and for the faction. If there's a cap of 30, but no detectives on shift/allowed to be on shift, and a situation arises where a detective is needed, then this will feel extremely unrealistic and the whole idea in my opinion does not make any RP sense.
If you get arrested everytime you evade cops, find other ways / escape routes. You can't lose ground units? Take sharp turns and fake turns, can't lose the heli? Lose ground units then go under a bridge. You just have to outsmart them, others do it everyday..