While I appreciate the French tradition, cassoulet is well known to be different from one area or France to the next. The reason being: you use what you have. The French have Tarbais beans and ducks...Americans have Great Northern beans (or Cannellini) and Chickens. If you're going to follow the tradition, you would use what you have. I do have access to ducks, so I use them...but I have heard using chicken confit in duck fat is outstanding for cassoulet and few can tell a difference.
I have made cassoulet for dozens of people each year the last 10 years. I once paid up for Tarbais beans and found they added nothing to the dish over Great Northern. When you combine all those flavors, the taste of the beans is lost...confit, pancetta, sausage, and other flavors dominate. So ever since I've used Great Northern and saved the huge money it costs to get Tarbais.
If I could find Tarbais at a local store and if the price was reasonable, by all means I'd give them another try. But buying $50-60 worth of beans for 30 people is silly in my opinion.