Preheat an outdoor grill for medium-high heat. Lightly oil grill grate, and place about 4 inches from heat source.
Lightly brush the swordfish steaks and mango slices with 1 tablespoon olive oil.
Cook the swordfish steaks on preheated grill until lightly browned on both sides, turning once, 5 to 7 minutes per side. After turning, arrange mangos slices over the steaks, and cook 5 minutes more.
Toss the spinach, tomatoes, and kumquats together in a bowl. Whisk together 1/3 cup olive oil and balsamic vinegar in a small bowl until well blended. Pour over the spinach mixture and toss to coat evenly. To serve, divide the salad among 4 serving plates, and top with swordfish and mangoes.