Preheat oven to 350°. Grease a 9″x13″ baking dish and set aside.
Bring a large pot of water to a boil. Add the pasta and cook for 5-7 minutes, until not quite al dente. Add the chopped cauliflower and cook another 2 minutes. Drain and set aside.
Meanwhile, melt 6 tbsp. butter over medium-high heat in a large saucepan. Add the flour, 1 tbsp. at a time, whisking constantly to incorporate. When the resulting roux is smooth and not lumpy, start adding the milk. Drizzle the milk into the pan in small splashes, whisking constantly, not adding more liquid until the previous splash has been incorporated. The roux will gradually loosen and begin to absorb more liquid at a time. When it is completely loose and soupy, add all the remaining milk at once.
Reduce the heat to medium and cook the sauce until it thickens, whisking regularly, about 5-8 minutes. Stir in the salt, white pepper, garlic powder, and hot sauce. Stir in 4 oz. of freshly shredded parmesan. Set the béchamel aside to cool slightly.
When the bechamel has cool about 5 minutes, stir in the pasta, caluliflower, cheddar, Swiss, colby jack, and 4 oz. parmesan. Transfer the mac and cauliflower to the prepared baking dish.
In a food processor, combine the Ritz crackers, remaining 1 tbsp. butter, melted, remaining 1 oz. shredded parmesan, and the Cajun seasoning. Pulse until the crackers are reduced to coarse crumbs.
Distribute the cracker crumbs over the mac and cauliflower.
Bake for 25–35 minutes, until the cheese is bubbly and the cracker topping is lightly browned.