Discard any clams or mussels that do not close to the touch. Scrub them well under running water and pull the beards from the mussels. Place the clams in a bowl of water and the mussels in a dry bowl and store in the refrigerator until needed.
Rinse the chicken pieces and pat dry. Sprinkle with salt and pepper to taste. Place a large, deep sauté pan over high heat and pour in enough olive oil to film the pan bottom lightly. Add the chicken and brown well on all sides, 8-10 minutes. Using tongs or a slotted spoon, transfer the chicken to a plate; set aside.
If needed, pour a little more olive oil into the oil remaining in the pan and place over medium-high heat. Add the sausages and cook, turning, until firm and browned on all sides, about 5-8 minutes. Using tongs or a slotted spoon, transfer the sausages to the plate with the chicken. Cut the sausages into chunks and set aside.
Add the shrimp to the same pan and sauté over medium-high heat, stirring briskly, until pink and beginning to curl, 2-3 minutes. Again, transfer with the tongs or slotted spoon to the plate holding the other ingredients.
Add the onions to the oil remaining in the pan and sauté over medium heat until softened, 5-6 minutes, adding a little more olive oil if needed to prevent scorching. Add the garlic and bell peppers and sauté until beginning to soften, about 5 minutes longer. Add the tomatoes and cook, stirring, for 3 minutes longer. Stir in the rice, pour in the chicken stock and add the saffron. Bring to a boil, then reduce the heat to low and simmer, uncovered, for 10 minutes.
Add the browned chicken and sausages and the peas or beans, pushing them into the rice slightly, and simmer, uncovered, for 10 minutes.
Drain the clams. Scatter the clams, mussels and the reserved shrimp over the top, cover and cook until the clams and mussels open, 4-5 minutes. Discard any clams and mussels that have not opened.
Remove from the heat and let stand for a few minutes until most of the liquid is absorbed and the rice is tender. Serve directly from the pan or spoon onto individual plates. Garnish with lemon wedges and strips of pimiento, if using.