Augie, I think you are in the ball park. Esp with the pork and ribs. Figuring you will get around 6# of finished per butt on the pork and serving a 1/4 pound. With ribs, you generally get 10 servable bones per rack you are looking at 120 bones. I would cut those into 2 bone servings and you would have 60 servings. (and hopefully being 2 bones vs. 1, they will only take one serving.
What are you doing with the chicken shredding??? If so you should be fine there as well as you can probably get 12 pounds of meat out of 6 whole chickens. and at .25 per person you will be at 48 servings.
One flat may not be enough.
Of course they will not all take a serving of each, but then you never know.
Have fun whatever you do.
KCBS CBJ 9831
Large Spicewine, Backwoods Smoker (Party), WSM, Oklahoma Joe's Offset, Wood Fired Oven