Everything that has been said is very good advice so far. As bill said, choose your background. A lot of the work that will have to be done (hair) will depend on the background, if it is a solid color then you can just drag the individual shots of each model to there position and use a detailed layer mask, with channel masks for the hair. If the model has frizzy hair, stand her up and place her, in the composite, where there will be nobody behind her, makes less crap your digital tech will have to deal with.
You might try using the butterfly lighting technique (your main lights) with a 1&1/2 stop fill (1&1/2 stop less than your main lights, I'm not trying to be rude, just as detailed as I can, cause I know I am a bit confusing). With this lighting you wont have to worry bout hard shadows cast directly behind the front models and, unlike a straight overhead lighting, you won't have to deal with the shadows under the eyes and such.
I would definitely shoot each model separately (less crap for your digital tech to deal with). I would also shoot a final composite shot first so that you can pose each of your models the way you want them to look, it will help a lot to compare between your composite shot posing and your individual shot posing. Like it has been said, try to keep your camera settings(aperture, shutter speed, ISO, camera height and location, focal length, these are your main concerns).
ok so that was more than a mouthful, hope it helped