Are you a bit of a pushover that you even consider such arrangement. Is SO feeding you stories? Are you very young? Since you still have curfew at your parents, you must be much younger than SO. Maybe your age and lack of experience doesn't let you see how ridiculous it is that he even suggest that.

Why can't she rent a small apartment if she makes slightly above average? She CAN. Sure it might not be fancy place but she can find one. She wants to save money living with you. Hm. Must be nice.

What confuses me though why are you even considering it. I honestly have never heard of a woman being ok with her SO bringing his female coworker living with them. He is 30. Grown man. Why is he behaving like a teen.

Make sure you had serious discussion with him. Otherwise he will move her in
and it will be a disaster.