Bring a large pot of salted water to a boil. Add the macaroni and cook as the label directs until al dente. Drain and let cool.
Toss the macaroni with the tomatoes, radishes, bell pepper, scallions and cucumber in a large bowl.
In a small bowl, whisk the mayonnaise with the vinegar, mustard, sugar and garlic salt. Add the dressing to the macaroni and vegetables and toss to coat. Season with salt and pepper. Refrigerate until serving.