Seriously, fruitylips has some very good suggestions here on how to order pizza... and, specifically, how not to order pizza.
Personally, I always hated combo pizzas anyway. They always look like someone basically swept the counter and called it pizza topping. The more shit you pile on a single pizza, the more likely it is at least someone will not like something that's on there, or, as fruitylips noted, that you will achieve a pizza no-one in the room is willing to eat.
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In general, when it comes to food, I'm pretty easy to get along with.
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Personally, my strategy is "If you don't already know the preferences of the people who'll be eating, ask them."