You’ll have plenty of potato salad and slaw. Depending on the mix of adults to kids, you might want to scale up on the hamburgers and down on the hot dogs a little, or just up on the hamburgers. We’d increase the number of tomatoes a bit in either case, but 25 onions, which can be sliced more thinly, are plenty.
Watermelons vary from soccer-ball sized to huge. The large, elongated ones weigh in between 15 and 35 pounds. Assuming you find 25-pound ones, we don’t think you’d need 250 pounds of watermelon, especially since it is not the only dessert item. We think a half-dozen 25-pound watermelons would be enough.