In a medium bowl mash half of the beans with a potato masher or pastry blender until well mashed. Add remaining beans, corn, corn chips, rice, onion, the 1/4 cup salsa, the chipotle peppers, cumin, and garlic.
Shape mixture into four 3-1/2-inch patties, about 3/4-inch thick. Place patties on a tray; cover and chill for at least 1 hour before cooking.
Brush both sides of patties with olive oil. Cook in a 12-inch skillet or on a griddle over medium heat for 10 minutes or until heated through, turning once. (To broil, preheat broiler. Place patties on the unheated rack of a broiler pan. Broil 4 inches from the heat for 10 minutes or until heated through, turning once.
Place some shredded cabbage on each tostada shell. Place the burgers over the cabbage and top with additional salsa, sour cream, cilantro, cheese, and avocado.