Strip the peel from the grapefruit with a vegetable peeler, taking care not to include the bitter white pith. Warm the grapefruit peel, ginger, and olive oil in a small saucepan over medium heat.
As soon as the oil starts to bubble, after about 2 minutes, remove from heat. Set the oil aside to steep for 30 minutes. Strain and reserve the oil. Meanwhile, rinse the quinoa in a bowl and drain. Put the quinoa in a small saucepan with the water and 1/2 teaspoon salt.
Boil over high heat, and then reduce heat to maintain a gentle simmer and cook, uncovered, for 15 minutes. Set aside off the heat, undisturbed, for 5 minutes. Transfer the quinoa to a bowl and fluff with a fork. Cool.
Segment the grapefruit over a bowl, reserving the segments and juice separately. Whisk 3 tablespoons of the grapefruit juice with the vinegar, honey, and salt to taste in a medium bowl.
Gradually whisk in 3 tablespoons of the reserved grapefruit oil, starting with a few drops and then adding the rest in a stream to make a slightly thick dressing. Season with pepper to taste. Toss quinoa with the dressing, chiles, scallions, and cilantro. Season with salt and pepper, to taste.
Preheat a stovetop or outdoor grill to high heat. Thread the salmon cubes onto the skewers. Brush with the remaining 1 tablespoon grapefruit oil, and season with salt and pepper. Grill the skewers, turning as each side browns but the salmon is still moist, about 3 minutes.
Toss the grapefruit segment into the salad, divide among 4 plates, and top with the salmon kebobs. Serve warm or at room temperature.