Without having the worms analyzed, you won't know if they have more calcium or not. I just trust in the data but also provide my chickens with free access to oyster shell. I figure it can't hurt to add it to the worms. I think chick starter and even the crumbly bits left over from layer pellets is great for the worms. I've only been adding it the past couple months, but I think the worms may do better with at least some added in with the wheat bran. Another BYCer is a big advocate of adding chicken feed to the worms and I did a bit of searching and found a study that showed benefits of giving chicken feed to mealworms (I don't remember where I saw it or I'd post the link). Also, you can use a variety of vegetables, e.g. carrots, watermelon rinds, beets, and even some green leafy stuff. Be sure to monitor when you give anything new so that if they don't eat it, you can remove it before it rots. For example, they don't really like eggplants much. Overall, I think a varied diet is better than using only potatoes or carrots. The wheat bran they sell for horses is a great inexpensive source. I think a friend told me he gets it for less than $0.25/lb. here in Tucson and feed is expensive here.