Michelle...this may be the "ideal" in the modeling world, but it is very, very unhealthy. I am 5' 3" tall and according to the height/weight charts, 115 lbs. is the ideal weight for someone MY height, not someone who is 5 inches taller.
With a BMI already at 18, you definitely do not need to lose more fat. Forcing your BMI even lower than it already is can lead to health problems such as irregular periods or lack of periods as well as a compromised immune system, not to mention what starving yourself does to your metabolism. What you need intstead is to tone and gain more muscle which could actually mean an increase on the scale because with equal amounts of fat and muscle, the muscle will weigh more but take up about half as much space as the fat. To build muscle, you need adequate amounts of protein and calories.
Have a look through the exercise forum; you may find quite a bit of information on how to gain muscle without harming your body in the process.
FWIW, if I had a job that had standards that required me to put my health and wellbeing on the line, I'd be looking for a new job real quick. No amount of fame and fortune is worth ruining your health over.