So, I believe I've just come across a workaround for the recruitment being subordinates and being unable to use them as factor upgrade fuel. However, it requires you downloading the multiple marriages mod for this game. By marrying any non-unique character, it effectively nullifies the subordinate status, making them into married slaves, similar to the all children are heirs sub-mod contained within. I forget the name of the mod itself, but a poster on here can likely reply with it's name. If I can find it, I'll edit and post it.

Edit: The mod is called the SFCRevamp mod, I believe

 If you choose to install the SFC Revamp mod, made by ChrisOwens68, hold the windows button, press the r button, and type %rundata% the go to appdata, roaming, striveforpower2, mods, then drag and drop it in from the file explorer you use to run the game. This is assuming you're unfamiliar with how to run the mods. It's somewhat explained in my old post 250+ days ago titled "Polygamous Marriage?" I believe. For those who already know how to install them, I need not explain.