Yeah they're not really "wild" or "feral", more like abandoned castaways who almost got wiped out due to tech supplies being cut off and imminent crisis linked to their reliance on tech for feeding and reproduction. Otoh they became inependent, and they DO have big brains. I don't want to spoil too much, though.
About the slavery - who can be enslaved and who cannot is my arbitrary choice. I have no time to write a separate story on why this or that of a couple dozens enemies who cannot be enslaved, cannot be enslaved.
Deep ones, for one, would make shitty slaves - they're alien, they are religious fanatics, they hate all land dwellers, they don't like to work (unless inspired by their Gods), they're stupid, they're hard to communicate with.