Please don't slip them in at night, that might go bad. I start my integration by putting together a separation pen and penning the chicks where they can be seen for about a week, than I start letting the chicks mingle with the adults, when things get rough I return the chicks to their pen and try again the next day. The first few times can last minutes. Eventually everyone leaves the chicks alone mostly, except for an occasional peck. Hopefully you have plenty of room and have places the chicks can hide and get away from the adults. I lock my chicks up at night separately for a month or so until I feel comfortable leaving them full time with the adults.