Bring poaching water and its salt to a gentle simmer in a saucepan. Add shrimp and cook for 2 to 4 minutes until opaque and at least 145°F, then drain and cool briefly in ice water before draining very thoroughly.
Boil ditalini in water with the tablespoon salt until fully tender. Drain, rinse under cool water and drain thoroughly.
Whisk mayonnaise, yogurt, lemon juice, zest, dressing salt and pepper. Fold in pasta, shrimp, celery, radishes and chives.
Cover and refrigerate for at least 30 minutes; the listed prep time includes this chilling.
Stir gently before serving cold and refrigerate leftovers within 2 hours.