Here is idea: slave should be generally less rebellious than free girls, who should in turn be pain in the ass. Reduce percentage, they rebel significantly up (free girls), they refuse to work on lot more occasions. As to why not brand every girl a slave, answer is simple: put in taxes. Taxes for slaves: say for every 20 slaves town/courts or police take 1% of your earnings, this can go up to 90%. So you can have 1800 slaves, but you would get amount of money the same as if you have 180 slaves and don't pay taxes at all. My math could be wrong, but now it becomes arithmethical problem instead of existental one.
As for "free girls", those you capture (customers daugters) shouldn't be automatically branded as slaves, but as "working under contract" - they legally couldn't go free because of debt, but they would earn some percentage of the money (less than wandering free girls, but still significant and as far as refusals go they would still be as bad as free girls).
As for captured monster girls, there should be some similar catch as for customers daughters above.