I just moved from the northwest corner of Arkansas a few months back. You might want to update your profile. Don't you just love those semi-domesticated deer (goats). You certainly have to take them into account. It's remarkable how much trouble they can get into.
I can understand why you want an 8' x 8', that's the dimensions of standard building materials. What do you plan to use to build the coop? Will it be solid paneling? Chickens can create a lot of dust but if they have access to the rest of the inside of that barn they will be hanging out in there and pooping on any equipment or whatever else you store in there. Maybe something for you to think about.
Anyway, if that barn does a decent job of blocking wind and rain, you might consider enclosing the coop with wire instead of plywood or some solid paneling to help reduce cost. A lot of that may depend on what that barn looks like, but that would take care of any ventilation issues. Again something to consider.
Is an 8x8 big enough fro 20 chickens? A lot of that depends on how you manage them. If you let them out at the crack of dawn every day of the year, you will probably be OK from a behavioral aspect. If they are locked in there for long periods of time you may have issues. How much do you enjoy sleeping in of a morning, especially if you have the flu?
Another issue is how do you lay it out in there? Those goats will eat eggs too, not just feed. How do you get enough roosts, nests, and food in there without them pooping in the food. Hopefully you can water outside. You can do it but it may be a bit tight.
You have Silkie mixes and a rooster. Will you be hatching chicks? If so you may wind up with more than 20 chickens. Silkies tend to go broody a lot. Will you be using a broody buster? Where can you put that so it will be predator proof?
Speaking of predators, what is your plan if you find you have a fox, bobcat, or coyote eating your chickens? Where are you going to put the chickens while you deal with the predator, that can take day or even weeks.
Can you make an 8x8 work? Probably. But it leaves you no flexibility in dealing with any issues that pop up. The additional flexibility is why I'd suggest you go with something bigger.
Good luck!
