1. To make chipotle syrup, in a small saucepan over medium heat, combine the ingredients and bring to a boil. Cook until it becomes a thin glaze, about 5 minutes. Set in the fridge to chill.
2. To make guacamole, cut avocados in half lengthwise. Remove pit and, with a spoon, scrape flesh into a small bowl. Add remaining guacamole ingredients and mix together with a fork, lightly smashing avocados. Set aside.
3. Preheat an outdoor grill or grill pan over high heat. Using tongs, dip folded paper towels into vegetable oil and rub over grill. Season fish with salt; cook on both sides until cooked through, 5 to 7 minutes total.
4. To serve, lightly toast the tortillas on grill or pan. Layer tortillas with cabbage, guacamole, and fish. Sprinkle with cheese, and drizzle chipotle syrup over the tacos.