In a large skillet, heat the vegetable oil. Add the pork and cook over moderate heat, breaking up the meat with a wooden spoon, until the meat is cooked throughout, about 5 minutes. Stir in the scallions and crushed red pepper and cook over moderately high heat for 2 minutes. Stir the sauce well, then add it to the pork. Stir-fry until simmering and thick, about 3 minutes. Reduce the heat to low and carefully fold in the tofu. Simmer very gently until the tofu is hot, about 3 minutes. Serve over rice.