Firstly, be careful about fertilizing your cacti. They don't require much and you could kill them with too much. I would not use soil with additives for cactus. They grow in very depleted soil, naturally, and its very easy to overdo it and kill the plant.
Next, it is my opinion that any nutrients a plant takes up from the soil are turned into good stuff (scientific terminology) inside the plant and not longer are fertilizers. So as good stuff it is not harmful to the tortoise eating the plant. I'm not a scientist, but this is what I think.