In a medium bowl, mix the onion, jalapeño, lime juice and garlic for the pico de gallo; season with salt and let stand a few minutes. Stir in the tomatoes and cilantro and season with the pepper sauce.
For the tacos, in a large skillet, heat the oil, two turns of the pan, over medium high and cook the onion, jalapeños and garlic, stirring often, until they soften, 7 to 8 minutes. Add the tomatoes; stir a minute or two. Add the stock, then the chicken. Add the milk, Worcestershire, smoked paprika and cumin; season with salt and pepper and cook, stirring often, until the chicken is heated through and finely shredded, about 3 minutes.
In a large, dry skillet over high heat or directly over the open flame on a gas stovetop, cook the soft tortillas, turning occasionally, until charred in spots, about 2 minutes. Top with the cheese. If using taco shells, preheat the oven to 350°. Place the shells on a baking sheet. Divide the cheese among the shells; bake until toasted and the cheese melts, 3 to 4 minutes. Fill the tortillas or tacos with the chicken, lettuce and pico.