Welcome to BYC! Please make yourself at home and we are here to help.
Yay! Those squatters will soon be layers! Yes, get them calcium right away and make sure they have layer feed and water.
Like Two Crows said, it's not good for them to sleep and poop in the nests because then that discourages them from laying in there. Make sure the roosts are higher than the nests, (chickens sleep in whatever is highest) put fake eggs in the nests so they see that's for eggs only, hang curtains or simply block off the nests in the evening. (You'll have to open them up bright and early in the morning though as morning is the top time for a hen to lay.)
Feel free to ask any other questions!