Looks like the built-in fighters are adding in a fixed OP cost (the reason it's not 20 is the Compact Automation hullmod reducing the OP cost by 5).
Maquech (TW) and Radiant (TW) carriers having a cost for their Nimbus fighters is a deliberate design choice, as those are the only Trinity Worlds ships that can use other fighter types. Giving their pseudo built-in fighters a cost of 0 would require lowering their fitting elsewhere to not give them an excessive amount of OP, but that would in turn mean the carriers would have a very hard time using any other fighter types that cost OP and still have anything left over for hullmods and weapons. A player who doesn't want to spend the OP on Nimbus fighters for their TW carriers can replace them with cheaper stuff like Talons or Mining Drones and get the OP refunded.
I swear I used to be able to put remnant subsystems on non-remnant automated ships with this mod but seemingly since the last patch I cannot put eg Adaptive Entropy Arrester on RAT Abyssal automated ships such as the Sariel or derelict automated ships like Rampart or Object 361 (from HMI).
I can't see a change like that in the patch notes so idk if this is intended behaviour or not?
It's possible that I'm mistaken that you could do this before but I'm pretty sure I used subsystems on some automated ships added by Knights of Ludd before along with abyssal & derelicts without any problems.
Abyssal ships can only take the Adaptive Entropy Projector by default. Anything else usually requires either an Abomination Interface or Synthesis XO. A few other manufacturers such as the Dustkeepers and Volantian Remnant Conversion get innate comparability as well, along with some individual ships such as Object 361 and the Timeless prototype from Tahlan, but the list hasn't gone through a major change recently.
Got Report that there is no option button even you got both Signal Masker in cargo, and Unique Ship in fleet.
Tested by All English enviroment with newest version and console code to spawn item & ship, no button either.
The only time where the Signal Masker button will show up is if the current ship has the "ship_unique_signature" tag that would cause the fleet to be identified even when its transponder is off. Many unique ships don't actually acquire this tag until after they're salvaged through normal gameplay, so for example if you just console command in a new Ziggurat that won't actually have the tag and can't receive a Signal Masker. The way to test this is to have only one unique ship in fleet and go dark, then run into another fleet. If you get the message that you can be identified anyway due to the presence of a unique ship and you still can't install the Signal Masker, then that's a bug and I will look into it.