You might be able to find some clues in things that are important to the sorority. I know that foods with symbolic meaning (the food itself, color, popular food from the area where the sorority was founded, etc.) are used at many pref ceremonies. Desserts are an obvious place to play with colors and symbols.
Obviously, finger foods are easier if it's a mingling-type event. Toothpick-friendly foods will keep fingers clean. You can also look to Tapas or Dim Sum restaurants for ideas - having several different "dishes" with the expectation that people will sample a few bites from each. That can help solve any special dietary concerns, such as a vegetarian, gluten-free, diabetic, or food allergies.
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