The theoretical and actual answers to you question differ. Let me explain:
Disney will automatically add a gratuity of 18% to all table service bills for parties of 6 or more people. So, since you're a group of 14, your question is really moot.
The theoretical answer to your question is that, were you a member of a smaller party, you would be free to tip whatever you felt was appropriate for the level of service you received. Moms Panelist Bret describes this very well
here. There are some people that think servers at buffet-style restaurants should be tipped slightly less than those at a full-service restaurant. I do assure that the Disney buffet servers are working very hard for you: taking drink orders, refilling drinks, bussing plates, offering food advice, and just generally providing Disney charm. I almost always think and 18% tip is in order at the buffet restaurants.