Fill a large pot with lightly salted water and bring to a rolling boil over high heat.
Once the water is boiling, stir in the medium shells pasta and return to a boil.
Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite; about 11 minutes.
Set a colander in the sink; pour in pasta; drain well; rinse with cold water to stop the cooking process.
In a blender, combine the mayonnaise, salad dressing, Italian seasoning, garlic powder, cayenne pepper, salt, and pepper; blend until completely mixed.
Place the coleslaw mix, tomatoes, cucumber, tuna, Cheddar cheese, Monterey Jack cheese, green onions, and the cooked pasta in a large bowl; pour mayonnaise mixture into the bowl; toss to coat.
Cover bowl; place in refrigerator until ready to serve cold.
