Prepare the salsa by mixing in a bowl the apple, cucumber, tomato, bell pepper, and onion (if using). Add the vinegar and sugar, season with salt, and reserve. It can be prepared hours in advance to intensify the flavors.
In a small bowl, mix the yogurt with the lime juice, season with salt and pepper, and reserve. In another small bowl, combine the chipotle powder with the orange zest and sugar. Rub each piece of salmon with 1 teaspoon of the olive oil and then with the chipotle–orange zest mixture. Let stand for 5 minutes.
Heat the tortillas either in a low oven, wrapped in foil, or on top of a flame (my method of choice), until they are softened and heated through.
Meanwhile, heat a grill pan. Season the salmon with salt and grill over high heat until nicely browned and just cooked through, about 3 minutes per side. Gently break each piece of salmon in half. Spread the mashed avocado on the warm tortillas and top with the salmon, Apple- Cucumber Salsa and the cabbage. Drizzle each taco with the lime/yogurt and serve right away.