Bring 1 cup water and 1/4 teaspoon salt to a boil in a small saucepan over medium-high. Stir in couscous. Cover, remove pan from heat, and let stand 10 minutes. Uncover pan; fluff with a fork.
Coat chicken thighs with cooking spray; sprinkle evenly with 1/4 teaspoon salt and 1/2 teaspoon pepper. Coat tomato and peach slices with cooking spray. Add chicken to grill; cook 4 minutes on each side or until done. Add tomato and peach slices to grill; cook 1 to 2 minutes on each side or until just slightly tender.
Combine remaining 1 tablespoon water, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, and sour cream in a small bowl, stirring with a whisk.
Cut peach slices into wedges. Place 1/2 cup couscous and 1 chicken thigh on each of 4 plates; top evenly with peaches and tomatoes. Drizzle each serving with 1 tablespoon sour cream mixture. Sprinkle with parsley, if desired.