Bring vinegar, sugar, and pinch salt to simmer in small saucepan over medium-high heat, stirring occasionally, until sugar dissolves. Remove pan from heat, add shallots, and stir to combine. Cover and let cool completely, about 30 minutes. Drain and discard liquid.
Whisk oil, lemon juice, mustard, pepper flakes, and ⅛ teaspoon salt together in large bowl. Add cooled couscous, arugula, mint, peas, 6 tablespoons pistachios, ½ cup feta, and shallots and toss to combine. Season with salt and pepper to taste and transfer to serving bowl. Let stand for 5 minutes. Sprinkle with remaining ¼ cup feta and remaining 2 tablespoons pistachios and serve.
