If you are also serving side dishes figure around a 1/3-1/2 pound of meat per person. Keep in mind that with pulled pork you only get 40% finish weight once cooked. When cooking chicken for groups offering pieces or whole birds is a pain. Someone will always complain that they didn't get their favorite cut. So keep it simple, cook thighs. Even simpler pull the thighs and serve it along side the pork as another sandwich option. Going that route you could cook all the chicken and prok 2-3 days in advance and then reheat and serve. Then all you have to cook the day of is the sausage. Which will give you more time to enjoy the party.
Serve sauce on the side so people can pick and choose what they want.
As for the sausage, depending on the size I'd figure that each person gets one. There may be leftovers, but you wont get anyone complaining that they didn't get one.