In a 15-inch paella pan or 12-inch frying pan (with at least 2 1/4-in.-tall sides) over high heat, turn sausages occasionally until browned on both sides, 3 to 5 minutes total. With tongs, transfer to a board.
Reduce heat to medium-high and add olive oil to pan; when hot, add onion, bell pepper, and garlic and stir often until onion is limp, 4 to 5 minutes. Add rice and stir until it begins to turn opaque, 2 to 3 minutes. Stir in wine, chicken broth, paprika, turmeric, and saffron. Bring to a boil over high heat, then reduce heat and simmer, uncovered, stirring once or twice, until rice is almost tender to bite, 15 to 18 minutes. Add salt and pepper to taste.
Meanwhile, rinse and drain fish and shrimp; cut fish into 1-inch pieces. Pull beards off mussels and scrub mussels; discard any gaping ones that don't close when you tap their shells. Cut sausages diagonally into 1/2-inch-thick slices.
Gently stir fish, shrimp, and sausages into rice mixture; arrange mussels on top. Cover pan with foil or lid and cook until rice is tender to bite, fish and shrimp are opaque but still moist-looking in center of thickest part (cut to test), and mussel shells have popped open, 7 to 8 minutes.
Sprinkle evenly with green onions. Garnish with lemon wedges. Serve from pan, adding juice from lemon wedges and more salt and pepper to taste.