Well they usually don’t make it that long. I’d manage the chicks as planned and just keep an eye one her. The last sickly chick I tried to nurse was a cross beak and she developed a respiratory infection that spred to two of my other chicks also. I had to cull all three. However, I had a black Ameraucana in my last batch of chicks that was very slow to get going. I thought at first she was a failure to thrive. She was smaller than the Ameraucanas and she just seemed very weak. She finally caught up by herself and is doing well. They are all about 16 weeks old now.
Many people will disagree with my attitude so, if you want, you can start a new thread in the emergency/disease section. Others may help with a diagnosis and remedies.