Steam mussels and little necks in 2” of water until fully open. Remove from heat and reserve steaming liquid.
Heat olive oil in a wide skillet. Add garlic and sauté until fragrant and golden. Add baby clams and juice. Drain steaming liquid into the pan and simmer. While the sauce is simmering, remove clams from shell and chop to the size of bay scallops. Repeat with mussels if needed. When sauce has thickened, add squid, scallops, clams, and mussels. Simmer until scallops are white throughout. Season with salt and black pepper.
Bring water to boil in a large stock pot. Salt and cook pasta according to package instructions before draining. Pour sauce over cooked pasta and garnish with chopped parsley to serve.