You may not thank me for saying this, but mixing hens from several different sources is a recipe for problems. Every established flock has diseases floating around within it which result in no symptoms, but which can be very serious to hens from a different flock. Raising chicks from eggs (which can be bought-in) ensures that they will be at least moderately resistant to the "home" diseases, and is a good reason for doing so. It would also be a good investment to vaccinate new chicks against the usual range of nasties, but vaccines tend to come in packs designed to treat 500 or more chickens, so getting a suitable quantity can be tricky unless you know a friendly and helpful larger scale poultry breeder.
With your existing hens, if they aren't dead yet, they'll probably get over the alien infections; indeed clinically they may already be over them, but have been pushed into moult. Either way, layers pellets are not needed if they're not laying, and given time they may well start laying again.