Fry the bacon in a large skillet over medium heat until crisp and browned. Transfer the bacon to a plate lined with paper towels. When cool crumble and set aside.
Put the sweet potatoes and Yukon gold potatoes in 2 separate saucepans. Cover the potatoes with water (by 2-inches), add 1/2 teaspoon salt to each pot, and bring the pots to a boil over medium-high heat. Reduce the heat, and simmer the potatoes until they are just cooked through and tender. The sweet potatoes will cook in about 15 minutes, and the Yukon gold potatoes should be finished in about 12 minutes. Drain the potatoes, and allow them to cool.
Combine the potatoes, scallions, celery, and serrano chiles in a large bowl. Add the mayonnaise, mustard, tarragon, salt and pepper, to taste. Toss to combine. Taste for seasoning, and add more salt or pepper, if needed. Toss the potato salad with the arugula, and serve on a large platter, garnished with the crumbled bacon.