To continue to express this properly, I think we need to define the difference between free specialist and citizen specialist.
The free specialist usually comes into the city as a GP. I consider these to be an influence they've left behind on the city for all time, a lesson, a memorial building, a local historical inspiration, a song, a play... in short they've done something great and left a legacy that will assist the city for the rest of all time (thus why they can't be removed from one city, moved, and added to another.)
Right now, this is the kind of slot the slaves fill, and in this role they are a bit out of place.
However, the Citizen Specialist is a whole population dedicated to their specialized profession, be it priesthood, engineering, merchandizing etc... These are the population assignments that players have the option to control. Just saying to establish the definition for the sake of the conversation.
Now, optimally, we could have Settling slaves add themselves to the city as a population while somehow enforcing that they are SET here as Citizen Specialist Slaves and can be nothing else. But since we have no way to enforce that they MUST be nothing but a particular type of citizen, we would need to utilize the 'Free Specialist' slots. We can do this while counting them as proxy population, adding to the Free Specialist (settled slave) version all the added penalties of a population point. The only penalty we can't add is the extra food necessary to grow to the next population point, which is an acceptable degree of inaccuracy.
Optimally, we can program it so that 'free specialists' of the Slave variety, to suddenly become population points instead when Emancipated.