Heat the oil in a large, heavy pot over medium heat. Add the fideos and cook, stirring frequently, for about 10 minutes, or until well browned. Using a skimmer, transfer the fideos to a bowl. Add the onion, garlic, and pimentón to the pot and cook until the onion is beginning to soften, about 5 minutes. Add the tomatoes, breaking them up with your hands as you do so, and their juice, raise the heat to high, and cook, stirring frequently, until the tomatoes have broken down and the sauce has thickened, 15 to 20 minutes.

Meanwhile, combine the stock, wine, bay leaf, and saffron in another large, heavy pot and bring to a boil. Add the seafood, cover, and cook for 4 to 6 minutes, or until all the clams and mussels have opened and the shrimp are opaque. Transfer the seafood to a large bowl and add the shellfish cooking liquid and fideos to the tomato sauce. Add the fideos and cook, stirring frequently, until the fideos have absorbed a lot of the liquid and are soft, 10 to 15 minutes. Add the shellfish, simmer gently just to heat through, and serve.