It's a learning experience! But if you spend time on BYC, you'll notice the 4 sq.ft. inside and 10 sq.ft. outside is what's recommended, and that's the minimum if all the birds get along. Add any wrinkles to that - like integration - and you will need more. A lot of this is dependent on your breeds and personalities, too. Some get along great, and can survive on less space. Others get aggressive and you'll need extra space. It's hard to predict what situation you'll end up in, so it's always good to err on the side of more space, just in case. And if you plan to let them out of the run to free range, you still have to provide a run that's large enough for them to live in, because there will be times when you can't free range - for example while you are away, and don't want them eaten by wildlife.
Letting the chicks into the coop by themselves is a good idea. So they can get used to it. Be aware that when they first start sleeping in there, even if they are familiar with the coop, they won't know to go in there to sleep. You may need to teach them and put them through the pop door yourself, until they get the idea. They may also be afraid to go in there because of the older hens. So start doing that now, so they have enough time to learn before you go, if they are to sleep inside the coop when you're gone.
You don't need to make a smaller/narrower perch for them. They can use a wide one just fine, and they'll need a wide one when they grow up anyway, so there's no point in putting in a narrow one only to have to change it later.
Make sure you have at least 2 food/water stations that are very far away from each other, so the chicks can eat without being harassed by the hens.
There will always be *some* level of pecking and establishing of pecking order, but as long as there's no blood or injuries, they'll be fine.
Good luck!
Hopefully though, these will be okay together. My three new ones are: Black Australorp (who is the sweetest and smallest), Cream Legbar (also sweet) an Easter Egger who looks much like an Americana. She is the biggest of the little ones. My three big girls squeeze together at the end of a roosting bar each night.
