To prepare chicken: Combine chile powder, cumin, garlic powder, ½ teaspoon salt and pepper in a small bowl. Place chicken between sheets of plastic wrap and pound with the smooth side of a meat mallet to an even ½-inch thickness. Sprinkle the chicken with the spice mixture; set aside.
Zest and juice enough tangerines (or clementines) and limes to get 1 teaspoon zest of each and 1 teaspoon plus 2 tablespoons juice of each.
To prepare crema: Combine 1 teaspoon each of the tangerine zest and lime zest, 1 teaspoon each of the tangerine juice and lime juice, sour cream and ¼ teaspoon salt in a small bowl. Set aside.
To prepare slaw: Combine the remaining 2 tablespoons each tangerine juice and lime juice, ¼ teaspoon salt and oil in a medium bowl. Peel 3 tangerines and halve the segments. Add to the bowl along with cabbage, scallions and cilantro; toss to coat.
Coat a grill pan with cooking spray and heat over medium-high heat or preheat an outdoor grill to medium-high. Grill the reserved chicken until no longer pink in the middle, 3 to 4 minutes per side. Transfer to a clean cutting board; let rest for 5 minutes, then slice.
To assemble the tacos, drizzle tortillas with a little crema and top with chicken, slaw and a little more crema. Serve with tangerine and lime wedges, if desired.
