Being a complete novice to this type of photography, allow me to bring a fresh approach.
What I would do is change the venue to an area outside any building with a balcony. Get everyone (200 is not THAT many people) to assemble in that area. The photographer will be on the balcony (2nd or even 3rd storey). Outdoor sunlight will be much better than any indoor light or flash. Make sure it's a shaded area (on the shadow side of the building) otherwise the sunlight will cast harsh shadows. This also solves the problem of having to have tiers of people. Everyone will be looking up, which can be a fun and unique perspective. The face is the important thing, not the body.
Timing and coordination is important. Have a signal (eg whistle), and tell everyone that when they hear the whistle, they should all look at the photographer. Otherwise there will surely be one or two jokers talking or whispering to each other, and spoiling the shot.
Another option to consider, if you are willing to consider digital, is a panorama! You can take overlapping sections of the group, and stitch it into a panorama. Panorama Tools (freeware) allows you to stitch up and down as well as left and right, so you could divide the crowd into four or nine sections, take pictures of those sections, and stitch everything into one large, high resolution panorama. Everyone's face will be clearly shown. In fact, you could take a few shots of each section, and choose the best of each to incorporate into the panorama.
Check out
http://users.erols.com/maxlyons/ for more info, as well as the download links for the software.