I made a few minor replacements in these recipe. Used Greek yogurt in place of sour cream, pecans instead of walnuts, and apple cider vinegar (instead of say, raspberry vinegar or similar). Didn't find the dressing too tart at all. In fact, with the added sugar it's too sweet, so I would say the poppy seeds and a good-tasting vinegar are enough.
Overall, I liked the flavor combo of the sweet dressing, melons, nuts, and feta. The chicken is a bit of an after-thought, though it adds protein and makes the salad more filling. Good for summer.