It is not possible to have too much space for your chickens!

If they live in an enclosed area, expect it to have ZERO vegetation in no time. The more space they have the less likely they are to "over graze". My chickens have made a nice dust bath under the picnic table and in an area under a lilac bush by the deck. Other than those 2 places there is nothing that says "chickens live here" in the way of vegetation destruction.
Your chicks will be fine as long as they aren't too confined. My nine 3 Y/O hens STILL bicker over where they want to be on the roosts at night. There apparently are more and less desirable spots even with 2x the "suggested" space at 4' off the ground. I don't know about the BRs or BOs ( I have 2 WRs but at 15 weeks old they aren't going to be dominant over the older birds just yet). The BAs and EEs will be no problem, they tend not to be aggressive. The Rocks might be top of the flock but even if you had 9 of the same breed (assertive or docile) there WILL be a hierarchy. There WILL be one bird that is at the top of the pecking order and one at the bottom and everyone else arrayed between them. My advice is to make sure you have enough roost space at the highest level for ALL the birds. They are generally more comfortable on the top so if there is plenty of room there, they won't be pushed down to lower roosts.
Plan the windows so air can flow through the coop without flowing over the birds on the roost. Ventilation is paramount in keeping your birds healthy but should not entail a draft. They can stay nicely warm even in -20F if there is no draft.