Heat oven to 425°F. In small saucepan over medium-high heat, bring 6 cups water, bouillon and sazón to a boil, stirring occasionally. Remove from heat; cover pot.
Season chicken with adobo. Heat oil in large paella pan (about 15 inches) over medium-high heat. Add chicken to pan; cook until golden brown on all sides, about 7 minutes. Transfer chicken to plate. Add chorizo to pan; cook until golden brown, about 5 minutes. Add onions and garlic; cook until translucent, about 5 minutes. Add rice; using a wooden spoon, stir rice until coated with oil, about 2 minutes. Spread rice in an even layer. Pour reserved hot liquid over rice. Transfer chicken to pan. Bring rice mixture to a boil. Reduce heat to medium and simmer until the rice begins to peak above water level.
Scatter peas, clams, mussels, shrimp and calamari over rice. Transfer pan to oven. Cook until the shellfish open, the shrimp is opaque and the water is absorbed, about 12 minutes. Remove pan from heat; let sit 5 minutes before serving. Garnish paella with pimientos. Serve with lemon, if desired.