Nuggets: Bake according to package directions. Cut nuggets in half. Set aside.

Macaroni: Cook macaroni in large pot of boiling salted water until just tender but still firm to bite. Drain; transfer to very large bowl.

Melt 6 tablespoons butter in same large pot over medium heat. Add flour; stir 2 minutes. Gradually whisk in milk. Bring to boil, whisking constantly. Reduce heat and simmer sauce 2 minutes. Add all cheeses, 1 teaspoon salt, and 1 teaspoon pepper. Whisk until cheeses melt and sauce is smooth, about 2 minutes. Remove from heat. Season to taste with more salt and pepper, if desired. Mix cheese sauce into macaroni. Mix in chicken pieces. Mound mixture in 13x9x2-inch glass baking dish. DO AHEAD Can be prepared 2 hours ahead. Let stand at room temperature.

Preheat oven to 350°F.

Bake macaroni uncovered until heated through, about 30 minutes, or 45 minutes if made ahead.

For a fast, easy alternative: use your favorite boxed macaroni and cheese. Prepare according to directions. Mix in cooked nuggets and bake at 350° for 30 minutes.