Bring a large stockpot of generously salted water to a rolling boil. Add the rotini and cook according to package directions until al dente. Drain and rinse immediately under cold running water to stop cooking and remove excess starch. Shake well and let sit for 5 minutes to drain fully.
While the pasta cooks, halve the cherry tomatoes, dice the red onion, bell pepper, and cucumber into small bite-sized pieces. Halve larger mozzarella pearls. Drain the black olives and measure out the pepperoni and Parmesan.
In a small mixing bowl or mason jar, combine the olive oil, red wine vinegar, minced garlic, Italian seasoning, sugar, salt, black pepper, and red pepper flakes (if using). Whisk or shake vigorously until slightly thickened and emulsified.
In a large mixing bowl, combine the cooled pasta, cherry tomatoes, mozzarella, pepperoni, olives, red onion, bell pepper, and cucumber. Pour about three-quarters of the dressing over the top, then sprinkle in the Parmesan cheese.
Gently fold everything together with a rubber spatula or large spoon until evenly coated. Cover and refrigerate for at least 1 hour, preferably 2-3 hours, to allow the flavors to develop.
Remove the salad from the fridge about 10 minutes before serving. Pour the reserved dressing over the top and toss once more. Taste and adjust seasoning as needed before serving.