Place chicken in a single layer in a wide saucepan. Add pineapple juice and half each of the garlic and the salt. Pour in enough water (or sodium-free chicken broth) to cover chicken by 1 inch (2.5 cm); bring to a boil over high heat. Reduce heat to medium-low, cover and simmer until chicken is no longer pink inside, 15 to 20 minutes. Drain and transfer chicken to a cutting board; let rest until cool enough to handle, about 15 minutes. Shred chicken by hand or using 2 forks.
Meanwhile, in a blender, purée tomatoes, chipotle peppers in adobo sauce, tomato paste and remaining garlic and salt. Heat half of the oil in a saucepan over medium heat; fry onion and sweet pepper, stirring occasionally, until softened, 4 to 5 minutes. Add tomato mixture; reduce heat to medium-low and simmer, stirring occasionally, until thickened and reduced to about 4 cups (1 L), about 30 minutes. Stir in shredded chicken; simmer for 5 minutes. Cover and keep warm.
In the meantime, cut pineapple slices into 16 half-moon shapes. Heat remaining oil in a non-stick skillet (or grill pan) over medium-high heat; cook pineapple, turning once, until tender and golden brown around edges, about 5 minutes.
To assemble, spoon some of the chicken and sauce mixture and a piece of pineapple into each shell. Sprinkle with cheese and cilantro. Serve with lime wedges.
Per serving (each of 2 tacos): 374 calories, 29 g protein, 15 g fat (5 g saturated fat), 34 g carbohydrates, 5 g fibre, 72 mg cholesterol, 663 mg sodium