If I have a flock of 6 chickens can I gust by 1 from you or do I have to buy 6. Should I bring a receipt from the place I bought them to show I already have 6 chicks.

I believe you have to buy 6 off them first. Then after that you can buy individuals

It only is possible if they are from Deaton's store and this year.

You have to buy 6 chickens from that store in ordered for it to work

Yes, as long as you have the proof, receipt of six or more, you can buy one at a time after.