Good advice from aart. You can expect the baby to get pecked. But don't be too over protective. Some people in your situation have been surprised to see their small single hold its own against the bigger chicks and even, on some occasions, become quite the assertive one.
In other cases, an older chick may take the younger one under its wing, literally. You won't know how they will adjust unless you let them try. One peck isn't going to kill a chick, and it's quite normal protocol for the new comer to receive a peck on the forehead, and then all is forgiven. Chicks have an instinctive drive to want to band together as a single unit. By putting this off and being over protective, you are losing this window of opportunity.
In other cases, an older chick may take the younger one under its wing, literally. You won't know how they will adjust unless you let them try. One peck isn't going to kill a chick, and it's quite normal protocol for the new comer to receive a peck on the forehead, and then all is forgiven. Chicks have an instinctive drive to want to band together as a single unit. By putting this off and being over protective, you are losing this window of opportunity.