I'd use the lens that will give you a working distance that invokes the feeling you want in the photo. Generally, for myself, I'd default to the longest lens I could get away with given what is behind me. I agree with you that the 135 might be the best choice. However, I myself would probably show up intending to use the 180 if I had plenty of room behind me. When filling a frame with 12 people, there will likely be people near the edges of the frame. The less stretching the lens does at the edges, the better. The longer the lens, the less stretching, and the "flatter" and slimmer your subjects look.
That said, the normal lens is the most versatile and all around best lens in the system, IMHO, and would perform the job just fine. And you also get the feeling of being closer to your subject, even though it may be harder to compose to make it look "good." I find composition with long lenses to be very easy. Wides are hard for me.
Consider your vantage point. Do you want to be lower than your subject, half way up, or higher than it? It makes a big difference in the image.
Are you familiar with mixing ambient light and flash? It's an incredibly valuable skill for outdoor portraiture I would consider using fill flash on a stand. Diffusion is harsher. Remember that larger light sources in relation to the subject = softer light. A tiny flash on a stand way back will provide a harsh fill. You want a broad source as close to your subjects as possible in order to get the softest possible light. Bounce umbrellas are great. My favorite fill flash is when it is shot into a big bonce umbrella or a large piece of foam core.
Bring an incident meter to meter the ambient light and the fill flash. Don't use a digital camera for a light meter.
I would use T-Max 100 or 400, depending on the light. You will probably be fine with 100, but in medium format you can really afford to use 400 if you prefer a bit more D of F. Personally, I would try to not have blisteringly sharp surroundings, but I know you said you want wide D of F.
I would use a tripod.
I would tell everyone to wear somewhat uniform-looking clothing, i.e. not a uniform, per se, but have everyone wear a mostly dark or mostly light outfit.
Have people make eye contact with the lens, not with you.
Don't make a habit of always doing a count before shooting. It is almost a necessity with groups, but be sure to just grab some shots as well.
Contrast will depend on the light more than anything else, not on the film.
Ideally, you do a practice run a few days earlier with some friends, and see what you can do with some of the suggestions you have got here. Then proof your results and see if there is anything that needs changing.
I realize that I have stated this as commands. I don't mean them that way. More like a list of suggestions and things to think about. There is more than one way to melt an iceberg.
