Add onions to the bottom of a slow cooker. Nestle pork right on top of onions. Chop entire can of chipotles and add them to a small bowl. Reserve 1 tablespoon chipotle sauce, then add rest to bowl. Add Coca-Cola and brown sugar, whisk to combine. Pour mixture over pork, cover, and cook on LOW for 6 hours. Shred pork, removing any chunks of fat, and continue to cook on LOW for 1 hour.
While pork cooks for the last hour, make the slaw. In a medium bowl add reserved chipotle sauce, mayonnaise, reserved pineapple juice, and garlic. Whisk to combine then add pineapple, coleslaw mix, green onions, and cilantro then toss until evenly coated. Cover and chill until ready to use.
Lightly toast tortillas by placing them under the broiler until edges start to brown. OR toast them over a gas burner until edges are lightly charred. Add pork filling to tacos, top with slaw mixture, and garnish with cilantro. Serve immediately!