I agree with keeping it separated. Wait until you‘re sure it’s fully thriving again before you reintroduce it back with the others. Just a suggestion, since I’ve had chicks that would act fine for a day or so then begin to weaken again. That’s when I’d decide to keep them separated longer until I was sure they were going to be strong enough to hold their own around other chicks. You have another little one for companionship and comfort. That can help to encourage this chick to move and eat and drink more. I think what you’re doing is a wonderful and worthwhile effort.I have had 4 clutches of chicks but I haven't had to brood them myself for a couple years as I normally have a broody hen. This chick has been "dead" twice now that I had to blow into it's mouth and stimulate and warm it up for it to slowly come back. This time I will keep it separated with a tiny friend. I would like to see it drinking and eating alone for a couple days before I'll risk putting her back with the healthy ones. This was also the first time I've gotten shipped chicks.