Prior to starting your paella, soak the clams in salted water for a couple of hours to remove any sand. Discard any clams that do not close when you tap on them. Press together the shells of any mussels that are open. If the shell doesn't close, the mussel should be discarded (also toss any with broken shells).
Heat broth, wine and saffron together in a large pot and keep warm. Pat fish and shrimp dry with paper towels. Sprinkle with salt and let sit 10 minutes. Use a mortar and pestle to mash parsley, garlic, thyme and 1/8 tsp salt into a paste; stir in paprika. Add water if necessary to form a paste.
Heat 6 tbsp of the olive oil in a 15" paella pan over medium high heat and quickly brown the fish on both sides 1-2 minutes. Do not fully cook. Remove to warm platter. Reduce heat to medium, add remaining 2 tbsp of oil, onion, scallions and bell pepper to paella pan, and cook, stirring occasionally until the vegetables are slightly softened. Raise heat slightly, add tomato with its juices and cook, stirring occasionally, until it becomes sauce-like, 2 to 5 minutes.
Pour in the warm broth and wine and bring to a boil. Sprinkle the rice evenly across the pan. Boil for 3 minutes, stirring rice and rotating pan occasionally. Stir in parsley paste. Add all reserved fish (but not shrimp). Gently stir the fish to coat in the sauce. Taste for salt. It is important not to stir after this point. Lower the heat and allow to simmer until mixture is no longer soupy but enough liquid remains to continue cooking the rice (about 10 min.) Add extra liquid if necessary.
Tuck shrimp, clams and mussels into the rice, placing edges of mussel and clam shells so they open facing up. Cook, uncovered, for 15-20 minutes until rice is almost done. Turn the shrimp over as they start to turn pink. Remove pan from the heat and cover with foil. Let sit 10 minutes. Discard any mussels or clams that did not open. Garnish with lemon wedges and serve with alioli.