To cook potatoes separately: Steam potatoes whole if they are about 3 inches long. If they are longer, cut in half prior to steaming. Steam until tender when pricked with a sharp knife. Set aside to cool, and refrigerate until ready to use.
To cook eggs separately: Place eggs in a large saucepan, and cover with in inch of cold water. Bring the water to a boil, and turn off the heat. Let the eggs sit in the hot water for 10 minutes, and immediately place in a cold-water bath until chilled. Refrigerate until ready to use.
To cook potatoes and eggs together: Place potatoes in a large steamer basket. Set eggs on top of potatoes. Steam until potatoes are tender. When potatoes are tender, eggs are finished cooking. Cool potatoes to room temperature. Place eggs immediately into a cold water batch. Refrigerate until ready to use.
For the sauce: Stir together reduced-fat sour cream, mayonnaise, and mustard.
To assemble: In a large container (dishpans are good for this), lightly toss together potatoes, eggs, pickles, and scallions. Stir in half the sauce and mix gently. Stir in reminding sauce slowly, until you are happy with the consistency. Salt and pepper to taste.