This has the potential to be a very interesting story, provided that you don't let the smut overshadow the plot. Too many times great stories get turned into 100k words of PWP. Don't get me wrong, I REALLY like smut, but there's a story here too.
As far as new girls for Harry, I think that Pansy is highly under-represented in the 'girls Harry bangs' catagory. You do mention Pansy and Millicent being raped, but not them being killed. So, assuming they're still alive, they could be possible additions to Harry's stable.
Pansy I could see as a 'high end' sub, almost a switch, being able to top the other subs under direction, but not on her own initiative. That is of course if Harry can get her to overcome the abuse she recieved at the hands of the DE.
Millicent I can't really see as Harry's body type, but I could see her as some form of muscle. Particularly for Pansy as they have a lot of shared history to draw on.
Another possible addition could be Romilda Vane, if for no other reason than she hasn't been done to death in fanfics. She already has an interest in Harry, (or did in years past in canon,) and as Harry has had little actual interaction with her, she could spring something really kinky on him without being ooc.
Author's response
def not Bulstrode - have an ugly mental image of her. Pansy was a possibility, I havent completely decided which girls are in yet. Vane I havent even thought about. No idea what her personality is like.