Homemade snail cookies are a great food for them, though the goldfish will more than likely still eat it. The good thing is that they give variety for the golds and take quite a while to eat so the snail will most likely get his fill.
As for calcium supplements, the blocks used to condition water for turtles work great, as do vacation feeders or a bit of cuttlebone or eggshells (often put into the filter, but I find that putting pieces straight into the tank in a high-flow area works great as my snails like to munch on it too!). There are mixed reviews on the effectiveness of calcium-rich foods as it is not dissolved calcium, but whether the snails can utilize that calcium or not they still benefit from other nutrients in those...namely spinach, dandelion greens, kale, turnip greens, and so forth.
Good luck with your snail...I warn, they're like potato chips, you can't have just one!