One chick will be very hard to raise. They are social animals and even in the brooder, need companionship to thrive. You would be better off getting a point of lay so she is big enough to defend herself.
Cramped spaces are never good for integration. If they are competing for space, they will not take well to newcomers.
If any if your girls go broody, you can try giving her a fake egg and let her sit it for 3-4 weeks (so she's hormonally in the right place to shift gears from setting to mothering), then try to swap the egg with a day old and see if she accepts it, otherwise, I'd go for an older bird in a setup outside your omlet run and then try mingling them during free range after a few days and see how they react so you know where to take it from there.