I kept my chicks on starter/grower until they began to lay. Then I switched them to layer. I don't add any additional calcium since the feed I give them has all the calcium they need. (It's an organic feed). They also...
Also, you are right about your fears of predators digging. A raccoon would make very short time of digging under that. I doesn't matter that the youngsters peep a lot. Even if a chicken is quite, the predators will...
Congratulations!! I'm sure the others will be with their sibling soon. I think you are doing just fine. It's probably a good idea to separate her/him till the others hatch.
Blessings!!
Kelly