On PoliceRP you pretty much would need to start in the PD for any kind of law enforcing roleplay as most departments need a PD rank to join (if you donate for Gold and join FBI some will also accept an FBI rank). I'm not sure if maintenance really has an equivalent department, but for similarity to CI Military you might want either to play as a criminal (and rob a variety of establishments, resulting in large numbers of angry police officers) or rank up to SGT and join a tactical department like SWAT, SCO19, SRT etc. which would make you one of the people who move in against heavily armed criminals (and, naturally, would be quite heavily armed yourself.) If you want roleplay, State Police and its subdepartments do focus a lot on roleplay, but as law enforcement they will naturally find themselves in combat often too.
It's also worth noting that on PoliceRP, ranks in the PD that aren't command (up to Sergeant Major) aren't on any roster and are not held to any kind of activity standards, so these ranks can be kept even if you don't play on the server or on PD (although you won't be promoted in PD unless you're active). Criminal jobs are mostly free to join, with some level/donator locked ones, so you only need to be active on it if you choose to join an official family.