Preheat oven to 375°F. Butter large casserole dish or cast iron skillet.
Bring large pot of salted water to boil. Add pasta and cook until al dente, about 10 minutes. Drain and set aside.
In large skillet over medium-high heat, cook sausage until browned. Remove sausage from pan, drain fat off and set sausage aside. In same skillet, add onion, peppers and garlic salt and cook over medium-high heat until softened. Add garlic and cook for 1 minute. Remove onion and pepper mixture from pan and set aside.
Add butter to same skillet and stir constantly until melted. Add flour and stir constantly over medium heat for 3-4 minutes. Slowly pour in milk and whisk continuously until smooth thickened, about 4-6 minutes. Remove skillet from heat and stir in 4 cups colby jack and 1/2 cup pepper jack. Season with salt and pepper to taste. Add pasta, cooked sausage and onion/pepper mixture to cheese mixture and stir until combined. Pour into buttered baking dish.
In small bowl, whisk eggs. Add remaining cheese and crumbled biscuits and stir until combined. Pour mixture over pasta in the baking dish. Bake for 25-30 minutes or until golden brown and bubbly. Let stand for 10 minutes before serving.