In American English, "Fit" is not used to convey good-looking and especially not to convey beauty. "Fit" refers to being in good physical condition (athletic, strong, and robust). You would not call a girl "fit" because she has beautiful hair and lovely eyes. However, if the girl runs 3 miles a day and has a flat stomach, you can call her "fit" to describe her more-than-healthy condition.
Saying a girl is "beautiful" is a much better compliment. Unattractive people can be very "fit."
Regarding the sexual harassment questions, you need to ask your boss. Some companies are very strict and others are not. But it usually depends on whether you are bothering the girl when you say these things or wasting time flirting when you should be working.
Your boss will know your situation with your coworker and will be able to tell you best how to handle the situation.
Best advice: Use "beautiful" and tell her outside of work.