There's more to the pecking order than just sneaking in ten new hens under cover of night. When they all wake in the morning, they will all still be strangers and suspicious of each other. The pecking order will still be shaken up, and conflict will be part of this.
A better method would be a barrier that they can see one another but not make contact. After about three days, you can take down the barrier and let them do a little mash up and settle some of the rank lineup.
I'd then put the barrier back up to give things a rest, then repeat the process next day, and the next until all are sporting their new ranks in the pecking order and things are relatively peaceful.
If you have enough eggs with these remaining eight hens, it would be much easier, and also much safer, to introduce baby chicks into the flock of eight adults. If you brood the chicks in close proximity to the hens, integration will be very easy as the chicks will be considered flock from the beginning. In six months, you will have ten new layers.
I made only a brief allusion to the safety of introducing new chickens to your flock. Adult chickens should be placed under observation in a segregated area in case they carry a respiratory disease. Quarantine might save the rest of the flock from getting it. But it will do nothing to screen out an avian virus. These forums are heavily populated with threads bemoaning diseases brought into a flock of previously healthy chickens by importing grown chickens that are carrying disease but not necessarily symptomatic.
If baby chicks had been chosen as the way to expand a flock, so much grief could have been avoided because hatchery chicks can be generally relied upon to be disease free.
