Instead of having this focus on all the PO's that complain that they don't get corp. It should focus on the fact that this could actually be a cool addition that if used right could be very good.
I'd like to suggest a few things though. Each corporal should be able to form Units, with deputies and PO's. I know they already do this by putting other cops in the Unit channel, however an in game mechanic would be cool to support this. Each squadron, Alpha, Bravo, Delta, etc. would have different branches. Let's say Alpha could be Marksman branch. Any corporal in the Alpha squad, could make a unit with fellow officers in game. This could maybe make PO's temporarily being able to buy MXM's, until the unit is disbanded.
A few rules that wouldn't make it completely fucked and OP in game.
A max of 4 people per unit, including the corporal leader.
If a Sr.APD member comes online, a unit shall not be formed to gain advantages in situations, f.ex if there are 1 corp, 1sapd member and 4 PO's, these PO's won't become mini corps in a situation, such as a federal event.
Only one unit of a certain branch is to be formed each server. (could be like 3 branches with different perks, like you suggested)
If a unit is disbanded, the gear /perks should be revoked/removed/seized immediatly.
A unit CANNOT be formed in an ACTIVE situation, in an attempt to get an advantage over the counterpart.
If all members of the UNIT die, the unit will automatically be disbanded, and would need to be recreated.
Units can only be created every 35 minutes.
So essentially, your idea. Just, smaller and temporary. And to ensure a more powerful first wave in a fed, unless a sapd is on.