So far this is a great fic.
For girls for Harry's Slaves I whould think that the Patils whould make a good addition as there culture and the way the wizarding world views some women whould put them in the sub role.
Gabrielle and Flure whould be slightly dificulte as you whould have to get around the fact that Veela are probably doms seeing as they are purely sexual creatures to begin with. Mabey have them be doms unless they meet a master which turns them into subs.
And of coures Greengrass and Davis are always fun in any kind of paring with Harry because not much is known about them and you could do anything with them.