I remember doing my researches before getting my very first choox. I remember the long-suffering pet / animal feeds shop man eventually saying to me, "Sally, you have read
enough. Just go and get some chickens. You are ready!"
I didn't treat my houses with anything and was very lucky. We did get red mite eventually but only after a couple of years. I was also blessed in managing to fall upon breeders who didn't pass on scaly leg with their pullets. Later in life I was given choox with scaly leg and discovered how nigh-on impossible it is to eradicate.
Ventilation is important and varies according to the weather. So ideally a good air gap under the eaves (rainproof but not airproof) and a variable window opening (meshed to keep rats and foxes out) - eg, a sliding door on a chain or something. You probably can't give them too much ventilation in normal summer weather and it has to get very very cold before they want the window shut right up - depending of course on the size of the shed relative to how many birds are in it, as they will perch close together for warmth as the weather gets colder.
Your fence and foxproofing plans sound fine. You'll remember that grass shorts the electricity, so to keep checking the zap and strimming or whatever to keep the grass from shorting it out.
I have always put the new choox straight into the house and shut it up, letting them out into their run the following morning - maybe not too early but let them out by midday. They have almost always just put themselves to bed the next night - maybe the odd one or two need finding and placing on perches (and some may not perch and need lifting onto the perches) but mostly if they got up through that door this morning they will go to bed through it tonight.
I have not had ex-batts myself but my feeling is that it is always better to get all the new birds in one go and mix 'em up straight away; introducing new birds to an established flock is never easy and there is always some unpleasant bullying of the newcomers.
Others will offer more - and better - advice but it sounds as if you have made very good preparations, so now just get your girls and look forward to enjoying them and their lovely eggs!
