1 day ahead: Place skewers in water to cover, and let soak 10 minutes. Thread scallops and shrimp onto skewers. Brush with olive oil and sprinkle lightly with Grill Mix. Brush lobster tails and calamari with olive oil and sprinkle lightly with Grill Mix.
Grill calamari over high heat (400 degrees F to 500 degrees F) 1 to 3 minutes total, turning once, until just opaque (overcooking yields tough, rubbery meat). Grill scallops about 4 to 6 minutes, turning once. Grill shrimp 6 to 8 minutes, turning once. Grill lobster tails 8 to 10 minutes, turning once.
Cut calamari into ¼-inch rings and lobster tails into ½-inch chunks; combine with shrimp and scallops. Cover and chill up to 24 hours.
30 minutes ahead: Toss seafood with Summertime Citrus Dressing to coat. Mix in sliced fennel and toss gently. Just before serving: Garnish salad with fresh fennel leaves.