Coat a large nonstick skillet with cooking spray. Add margarine, and place over medium-high heat until margarine melts. Add shallots and garlic; saute until tender. Add mushrooms; saute 5 minutes or until tender. Add zucchini and green pepper; saute 5 minutes or until crisp-tender. Remove from heat, and set aside.
Combine milk, eggs, egg whites, pepper, and salt in a large bowl. Stir in vegetable mixture, bread cubes, and cheeses. Pour mixture into a 13- x 9- x 2-inch baking dish coated with cooking spray. Bake, uncovered, at 325° for 50 to 60 minutes or until lightly browned. Let stand 5 minutes. Cut into rectangles, and serve immediately.