Transform the ceviche from a classic seafood appetizer into a fun finger food by serving it with tortilla chips.
Combine snapper, lime juice, and orange juice in a small glass bowl, making sure that fish is fully submerged in the juices. Cover, and refrigerate 8 hours or overnight.
Bring a medium pot of water to a boil, and add shrimp. Cover, turn off heat, and let shrimp cook 2 minutes. Transfer to a bowl of ice water, and let sit 5 minutes to cool. Peel, devein, and cut shrimp in half lengthwise.
Drain fish well. Transfer to a large glass bowl, and add shrimp. Add salt, sugar, and oil. Fold in orange and grapefruit segments, jalapeno, tomatillos, and cilantro. Season with a pinch more salt or sugar if necessary.