Trying to figure out the cash system. Should I always be sending girls to different agencies, or is there any way to make money by keeping her at my own agency?
In my experience, send most to
Some Other Modeling Agency unless they match the criteria really well with the other agencies, which unlock as you progress playing the game.
Some Other Modeling Agency typically only pays a few dollars (useful if you pay a model the $10 fee) but will increase the pay out over time if you send them easy models.
Office Employment, Waitress Recruitment, and
Fitness Models are worth referring to but only if they like sucky sucky, are polite, low fat, respectively. If you send any agency a model that doesn't match the criteria it will decrease your relationship bonus i.e. the amount you are paid in future.
I haven't found other agency criteria to match well e.g. tall, fat, race, elf - so I don't refer models to them. It seems pretty RNG if some agencies send you a red email of fail so just send only the best criteria matches and don't worry about it.