Minor nitpick: Aria and Arizona actually enter your household as slaves (Assuming you actually pursue trying to buy Aria and then subdue Arizona), but can be freed to assume Paragon status. You can't re-enslave them though.
I added a note about those two in the post, but the main intention was to share the calculations for enslaving through dialog. Since those two become paragons once you free them, they follow the paragon logic for re-enslavement (meaning you can't, because they aren't Milou or Felix)
(except maybe Milou and Felix, didn't know they were even possible since they're paragons but rumor is they are).
I didn't include that part of the code because it's separated a bit from the main logic but here:
The way it works: If the NPC is a paragon (status 5), it checks if their ID is 40 (Milou) or 34 (Felix), and if it is, it sets a variable "parok" to true
The next line checks if the NPC is a mercenary (status 4) or if that "parok" variable is true, and if either is true it shows the Enslave option.
So in short, yes, Milou and Felix are hardcoded to be the only enslaveable Paragons