I would use a 400ISO film. Some time prior to the shoot make a test roll with a few subjects placed so they would represent the area that the group will be occupying. Try your different lenses. The results should indicate where reflectors would need to be placed and which lens gives the best composition. Contrast would be affected by the quality of the light and film development time. I prefer cloudy. From what I know about Colorado -- if you don't like the weather, wait five minutes and it will change. Take plenty of film as it is hard to concentrate on twelve faces at one time. Select the one that your boss looks best in!
I once did family pictures (my relatives) with 27 people ranging in age from an infant to one over 80. It was outdoors, overcast and three flash units to get even lighting. Before PhotoShop and digital days so you may be able to run your tests with a digital camera -- I had used Polaroid. I used 2 1/4 and 4x5 and had a friend press the shutter release so I could be in some of the pictures.