Red Snapper and Shrimp Ceviche with Avocado

Name: Red Snapper and Shrimp Ceviche with Avocado
Ingredients: 1 medium shallot, very thinly sliced into rounds; ⅓ cup (or more) fresh lime juice; ¼ cup fresh orange juice; 1 tsp. crushed red pepper flakes; 8 oz. best-quality skinless, boneless red snapper fillet, cut into ½ pieces; 8 oz. rock shrimp, halved lengthwise, or small shrimp, peeled, deveined, cut into ½ pieces; Kosher salt; 2 avocados, chopped; 1 mango, chopped; 1 14-oz. can hearts of palm, drained, sliced crosswise; 6 canned piquillo peppers or roasted red peppers from a jar, finely chopped; 1½ cups torn mixed tender herbs (such as basil and/or cilantro); 2 Tbsp. extra-virgin olive oil