That chick hatched a couple of days ago didn't it? If you are going to add chicks, do it now. It may or may not work, but the longer you wait the less likely it is to work. The clock is ticking.
As long as the chick is with the broody hen, it will be fine, not lonely at all. At some point the hen will wean it, abandon the chick and go to rejoin the rest of the flock. Sometimes with a single chick the hen and chick seem to form a bond and they never really separate. Sometimes the chick is left all alone to make its way with the flock. It will hang around the flock but sort of stay on the outskirts. A lot of that will depend on the personality of the other chickens and overall flock dynamics. During the day my juveniles often mix in with the adults quite well after the hen has weaned them. At night they usually sleep a certain distance away from the adults.
Will that chick be OK by itself? I'd be OK going that route. Chicken society can handle a lot of situations if we just let them.
Azygous, how does your single two month old handle the nights? Where does it sleep? Just curious.