White urates is most likely the mucusy white vent material, and is kidney waste. Pictures would help. I would continue to feed your chick feed until it is used up, and just provide the crushed oyster shell and crushed egg shells so they will take what they need. Then you can either buy layer feed which has 4X the calcium, or give them all flock/flock raiser feed which has a larger crumble and is good for roosters, hens, and baby chicks to eat since it doesn’t have too much calcium. Layer has about 16% protein while flock raiser has 20%, and the higher protein can help if there is any feather picking. They also seem to prefer the higher protein feed, and it is best to use when you have chicks, or non-layers eating it as well as layers.
Make sure that your roost is higher than your nest boxes, and at night move the girls from the nests to the roost to train them to use those. Covering the nest boxes just for nighttime may help, and uncover them early in the morning. It is not uncommon for hatchery birds to start laying at 17 weeks, although many wait until 20-23 weeks. Each bird and breed varies.