Cut sausage into 1/2-inch thick slices. Remove skin from chicken; trim off fat. Cut through each chicken leg at joint to separate thigh from drumstick.
In large paella pan or wide deep skillet, heat oil over high heat; cook sausage, stirring, until browned, about 2 minutes. Transfer to platter. Add chicken to pan; cook over medium heat until browned, 7 to 10 minutes per side. Add to platter.
Stir saffron into chicken stock; set aside. Add onion and garlic to pan; cook, stirring, until softened, 4 minutes. Add stock mixture, tomatoes and green pepper; bring to boil, breaking up tomatoes with spoon and stirring to scrape up any brown bits from bottom of pan. Stir in rice, salt and pepper. Add chicken; simmer gently over low heat, stirring often, for 20 minutes. Gently stir in peas and sausage.
Meanwhile, scrub clams under cold running water. Discards any clams with cracked shells or ones that do not close when tapped. Nestle clams in rice until almost covered; cook over low heat until rice is tender and clams are open, 7 to 10 minutes. Discard any clams that do not open. Garnish with onions and lemon wedges.