I found this an intresting take on the Sex Slave story and that it is a bit more sadistic then others but not too extreame to the point that it is wreid. all in all it is a good start and i have a suggestion and you don't have to follow it but maybe you could have some of the eventual slaves serving a specific function.
For example With cho who in most descriptions is tall and athletic and has long legs, if she becomes a slave she could be a sex slave while also being a ponygirl and you can make it that Harry makes collars that with runes connects the person to him but only he gets information. This way he will know the best way to punish unruly slaves and i say this as then it makes the sex slave more developed and have a hierachry that changes with Harry's interest.