I see lots of good advice, and I am not one to argue with portraiture experts. But, you mentioned something about shooting in the hallway. Ugh! I'm envisioning a corridor of lockers. For that situation, I might consider using the long telephoto zoom with large aperture to get the lockers out of focus. You would need a wireless trigger device though - maybe even on a more focused spot flash.

Post Edit: I reread your long email again and saw that you have a black background, so perhaps my suggestion above is not relevant. Sorry.

One big problem with your scenario is the ambient fluorescent lighting which will cause green color cast on the peoples faces. You could either get at 18% grey card exposure reading or use gels. PLease read this on the subject of "Gelling for fluorescent"
at the Strobist website.

http://strobist.blogspot.com/2008/04/lighting-102-61-gelling-for-fluorescent.html

my suggestion - simply used the monolight and thats all. You don't need to complicate things. Use your 18-5x lense and set your lighting to F8 to f11. Lighting dirctly in front of the subjects. Subjects at least 4 feet fron the background (prefer 6) and place your light about 6 to 8 feet from the subject. Light should wrap arond subject to eliminate body shadow. You could use a tripod (remember to turn off iS if you do).

Most important, whatever method you use, try it out first at the actual location. This eliminated or at least reduces problems and increases ones confidence.

It is to wrap the light around the subjet and light the background to reduce harsh shadows of the subjects bodt. if the light is directly in front of the subject, this will also reduce shadow. You could place your subjects very close to the background and live with a slight shadow/dark halow around them. what you do also depends on how clear do you want the butterfly background to show up. The further away from the background your subject is placed, the less light with fall on the background.

you could experiment at home using the background and using something to simulate the the people you are photographing such as a mop in a bucket with a blanket draped over it. Experiment to see what you like as far as distances and the relationship among the background, subject, light and camera positions. Also take you camera flash with you as a backup. Good luck

Before any one starts telling me I shouldn't do this--I know. I've been volunteered and didn't say no. So what I would like is some help on setting up. The event is a father/daughter dance at an elementary school. (Previous photographer can't do it and I believe she used on camera flash.)

The set-up will be in the hallway and I believe is lit with florescent lighting. I have a black background--it will have butterflies on it to follow the theme. The dads will be sitting on a stool with daughters next to dad. The shots will be full length so the girls' dresses can be seen. This is what the committee requires.

I have a Canon Xsi and a 18-55mm IS lens (also have a 75-300). I have a 430ex with a small plastic diffuser and a tripod. I also have a monolight with a soft box (son's cast-off.) I have no reflectors.

I have thought about using the soft box tipped to the vertical placing it adjacent to the camera with the camera at about eye level to the father. The soft box would be slightly higher than the camera with a slight tilt. Or I could just use my flash on-camera with the diffuser. I would also like any input you have on camera settings. I was thinking an using AV with f/11 to make sure everything is in focus. (and BTW--what should be my main focus point?)

I am worried about the school lighting and whatever I decide to use especially concerning color cast. I have no way to practice the exact lighting situation here at home and can't get into the school until night of. I am going to practice the different set-ups today when my friend brings her granddaughter over to see about hot spots or nasty shadows.

I guess my question is about the combination lighting. Do you experienced folks think this will work? Sorry if the post is so long but wanted to give you as much detail as possible in order to help any responses I might get. I will greatly appreciate any help given. TIA.