No life rule at all. The only rule is that crim and gov are separate (crim classes being anything that isnt a government entity). you can join as much or as little as you want, just each department has different PD rank requirements (some departments allow other department ranks to join). Basically your PD rank is the main requirement tho. Heres a list of all departments with a quick summary of what they do.
PD- join at any rank, start as an OFC and work your way up to PD command or join other departments (you can do both)- Patrols the city and assists other government with their operations. can do most of what other gov can do.
Dispatch- join at ANY rank though it is recommended you take PD training first. Apply on fourms or come to tryouts- RP oriented radio organization
EMS: join at SNR by applying on the fourms or coming to tryouts- RP Oriented medical branch
State: join at LCPL by applying on the fourms or coming to tryouts- patrols outskirts and does cool stuff
Secret Service: i think its CPL or LCPL to join . You can also join by achieving SSA in FBI- protects president, when no president is on acts as an investigative unit
FBI: have gold donator rank, and get trained as a PA. Investigation department who busts criminals for drugs, firearms, and other cool stuff. you can go undercover in this department. Really fun RP experience. also has HRT, their sub-tactical department who rescues hostages.
SCU: join at CPL in pd by applying- does investigations and busts as well as recon missions
Tac Departments: each of these departments deal with tactical situations normal gov cant deal with. they have increased armor, better weapons, and more. Join these by applying at PD SGT or coming to tryouts as CPL
ARU- Armed response unit , has a K9 class you can get at Detective Sergeant
SPRT- States tac department, can join at trooper FSTC
DS- Elite counterterrorism unit
SWAT- Special weapons and Tactics
SRT: ATF Special response team
DOC: prison guards. you do not have to be in state to join.