1. Rinse and pat tuna dry. Whisk together 6 tablespoons juice, 4 tablespoons each of mint and chives, and 2 tablespoons oil in a small bowl. Season with salt and pepper and divide mixture between 2 resealable plastic bags. Add tuna to one bag and zucchini and peppers to other bag. Seal bags and shake to coat tuna and vegetables with marinade. Marinate both at room temperature 20 minutes.Make salsa::
2. While fish and vegetables are marinating, halve avocados. Pit and scoop flesh from skin with a large spoon to a cutting board, discarding skin. Cut avocado flesh into 3/4-inch pieces and transfer to a bowl, tossing it with remaining 2 tablespoons juice. Trim ends of grapefruit. Standing fruit on a cut end, trim peel, and white pith away from sides. Cut grapefruit segments free from membranes and halve crosswise. Transfer grapefruit to bowl with avocado and gently toss with remaining 2 tablespoons each of mint and chives and 1 tablespoon oil. Salt and pepper to taste. Cover salsa with plastic wrap; chill until ready to serve.
3. Preheat grill. Drain skewers and thread each alternately with about 3 pieces tuna, zucchini, and pepper. Grill kabobs on lightly oiled rack over moderate heat, turning until golden and cooked through, about 12 minutes.
4. Serve kabobs over salsa and garnish with lime wedges. Makes 10 kabobs.