Catering party for 60-75, and am making one salad (tho 2 versions ate being made one with meat and one vegan..) How much salad do I make?

4 ozs per person

look at professional catering sites or other crowdsources. one example for serving 50+. buyfood.co.uk/cooking-recipes/cooking-for-a-crowd/salad/
You will notice differences in quantity between green salads and composed salads. also, depends (to some extent) on what else is being served at the meal. Good luck!