I have been trying to find and catalog all unique recruits in the mod. below is what I have.
You can open the Mod file in fcs and search "recruit" to get a list. The following are those I know of or have found in game. Any that are enemies must be in a prison cell to talk to.

Asheplate - found in Dust king tower - Dust king must be defeated and the person talking to Asheplate must have the torture dustwitch found on Dustking to recruit.

Goldshade - Black ninja tower - defeat Dimak and Buzon and the one talking to them must have the stolen ration pack found on Buzon I believe.

Indego Bull - Found in Shark bar - defeat either the leader of the red sabers or swamp ninjas to recruit.

Guilliman - Found in fishing village by cannibal plains. kill/capture Grand Wizard and Meatlord. He should be sitting on a bentch in the bar. If he isnt you can try soft reloading the area by saving and loading from the main menu. Hes a soldier bot.

Shaelzimar - Found in Admag in the building with Esta. Must specifically Kill the Pheonix. Has 105s in all stats and 200s in hp bars. Is "Stone Shek" race like the updated Seto, member of The Invincibles, and Esta.

Holly Warrior - Blister hill I think never got this one but I saw him in the game files. Is a rep recruit as far as I know for once you hit 75+.

The next 3 have a similar obtain condition based on killing the other the leaders of the Bandit triangle in the South East. The Terminator, Amethyst, and Skin Titan all require killing every leader that isn't the recruit faction's leader. Kill The crab queen, Elder, Savant, And the Reaver leader in Ark. Do not kill the leader of the faction you are going to recruit from until after you have your unique recruit. Skin Titan and Terminator have 105s in all stats and high hp pools Amethyst(a crab) has lower overall stats but an insane hp pool once fully grown. 1228 in all fields I think.

Tessgore - will be in one of the hunter assaults sent by the UC after attacking a noble. Must have Killed Eyegore (maybe just imprisoned not 100% on this one) to recruit. Shares the Race of Eyegore and can were his unique armor. Has 105s in all stats and is the same enhanced Southern Soldier drone race as Eyegore. Eyegore's assault may not always proceed the spawning of Tessgore's as an assault addon. It is possible to encounter this recruit able enemy without having trigger Eyegore's Assault in any hunter assault or elite hunter assault.

Horus - Unknown - listed as Horus Ruprical in the FCS he has no recruit dialog in the game files I can find. He is list as a tech hunter but I haven't found him in Worlds End, Black Scratch, Flats Lagoon, or Last Precipice. I have tried looking for him after having recruited Guilliman and also post killing every major faction leader that isn't Tech Hunter. I suspect he is not in the game.

There is also a unique recruit in the anit slavers I cant recall off the top of my head. I think you need to kill/imprison all of the meitou barring noble leaders to get him.