When to release ??

I don't put my 'chicks' with the main flock until they are about 10-12 weeks old, otherwise they are too small to deal with the push and shove of 10 other chickens.

Plus, I did read somewhere that they shouldn't perch until 10 weeks old (but I'm not sure how reliable that is)

I've had 2 lots of chicks this year in seperate runs etc until 10/12 weeks, without a mum, and they've been fine. They don't get lonely cos they have each other to play with.

Can they not run with the flock during the day and return to their ark at night? Is the ark within the main run or in a separate part of the garden? I generally let the mother hen decide when she is ready to leave them, and usually when the chicks are about 5 weeks or so the hen gets back into lay and then I put them back in with the flock, but keep the babies separate, but that's usually because I've got a bantam broody and the chicks are large fowl, so will not be kept together as adults anyway. They also have different feed requirements. I let my chicks grow up separately and then (if I am keeping any) they get introduced to whichever pen they are joining when they are almost adult sized, usually around 4 months or so.

If you are intending to keep all your chickens together in one big flock then you could return hen and babies whenever you wanted. Mother hen will protect the chicks from being bullied. I would place the ark within the main enclosure and you will find hen and chicks will probably go back there to sleep for a few more weeks.