Cut off and discard thick pummelo skins. With your fingers, divide pummeloes into segments. Pull off and discard membranes; pull fruit into 1/2-inch chunks. Place in a bowl.
Gently stir in cucumbers, cilantro, chili, and 2 tablespoons fish sauce. Taste and add more fish sauce if desired; if making up to 1 day ahead, cover and chill (salad will become juicier). Sprinkle with peanuts just before serving.