Heavy metals are toxic to all organisms, whether they are required micro-nutrients (copper, iron, manganese, zinc, nickel) or pollutants (mercury, lead, etc.). Studies determined that with respect to fish, lead was third highest in toxicity, mercury and copper being higher, and second highest (to mercury) for plants. Brook trout showed juvenile deformity with levels of lead beyond 0.058 ppm. It is known that some metals will bind to organic matter and carbonate ions and becomes less toxic, and other fasctors such as water hardness and pH also have an impact on toxicity.
Whatever the numbers, lead clearly has a toxic effect on plants and fish, and therefore should be kept out of the aquarium.
If the substrate will not hold down a plant, the depth is insufficient for a planted aquarium. Increasing the substrate is easy.