Here is what I would tell a family cook:
Unfortunately for the budget, shrimp on a buffet get gobbled at a remarkable rate, and it is very easy to run out. For your chosen combo, allow at least 1 pound peeled headless for each 3-4 people.

baked salmon, baked halibut- use the figures on the plan for 100 table for fish, about 60% salmon and the rest halibut and add 10% to each total because people take more when there are two choices.

caersr pasta salad, italian pasta salad-total of 6 pounds dry pasta for each 100 plus the additions. You decide how to split is.

fresh green beans- about 24-25 pounds as purchased per 100.

baby red potatoes w/butter, with rice, 25 pounds per 100
rice pilaf, with potatoes, 6-7 pounds dry per 100.