And here's another important reason why introducing a single chicken into an existing flock, no matter what age, is unwise. Chickens tend to focus their attention on anything that is new, unique and different. They want to investigate this unique thing unrelentingly with their beaks, and this can prove dangerous when the unique thing is a chick that is much smaller than the rest of the flock.
Chicks, as they develop, derive their self confidence from being part of a unit with other chicks of the same age. Depriving a chick of this will set it up for bullying, maybe for the rest of its life. You are doing a chick no favors by having it as a single. The only way it can work is if a broody has hatched and raised a single chick and protects it until it's reached significant size.
Please wait until it's feasible to add two or three chicks.