Are you seriously discussing this?
Sarah and Melanie aren't in the sales business.
Lily isn't a prostitute; she simply buys wholesale orders from Quinn for Pink Rose.
Quinn could have worked when they urgently needed money.
Otherwise, Riona would have been surprised by a threesome with her and Tremolo.
And I'm sure Quinn had more than five girls working for her. Due to the game's conventions, we only know about the HOTs girls.
Sara, Mel, Kami, and Rio receive free training from Quinn because they are HOTs. That's why the rumors spread. The other employees could have done it for a percentage.
I'm sure Gina or Tiffany could have easily worked for her. Seriously, consider that Gina didn't do anything on the exam but continued studying.
Kylie and Tara—they wanted to try, didn't they?
You don't imagine playing some RPG and arriving in a town with a population of just 20 NPCs, do you? There are thousands of them, but for the game, we only need these 20. If it were necessary for the game's story, we could easily expand Quinn's staff of dorm girls to 10-20 workers.
The game takes place in a state similar to the United States, but it's not. It's a fictional country for the game.