Crispy pan-fried tofu cubes can be used but not soft or medium tofu. The tofu should have a crisp exterior in order for it to withstand the force of the quick stir-fry method without breaking apart. You can also use cubed extra firm tofu. Toss it in some corn starch before pan-frying all sides until a crisp on the exterior and golden brown. Then use it for the stir-fry.