Building small is a great way to learn and not get discouraged by the size of the project. And these are just my opinions and suggestions.
I would build any earthen structure no less than 18" thick. But probably thicker if its going to hold a living roof AND animals. Foundation depth should go down to at least your areas frost depth if not a little deeper. Foundation thickness should be at least 2X as wide as your walls thickness. So if you build a 18" thick cob wall. your foundation should be 36" thick, 9" wider on each side of your cob wall. I would also start my cob walls at least 18" above the ground, built on top of stone, brick, urbanite ect. Anything that will keep your cob walls far away from any possible ground water.
Is the ground not a good enough place for goat play? The structure will need to be fairly robust in order to accommodate living roof and animals safely.
http://weblife.org/cob/ this site has the Cob Builders handbook from front to back for free. It will answer pretty much every question you have about said endeavor.
The less 90 degree corners your structure has the better it will stand up to seismic activity. Im not too familiar with the habits of goats and sheep, but the structure should hold up to them just fine.
I would look to vernacular roofs of your region to find something that works there.
And since financial resources are non-existent you'll have to use what's in your immediate vicinity to build with, which may end up not being cob. So in that respect be flexible and use whatever you have available. Whatever you have on your property, whatever you can go and scavenge, don't limit yourself! There is a plethora of building material everywhere, you just have to be open to see it. Whenever I see an abandoned pile of tires, I see a wall. Whenever I see discarded piles of concrete I see a rubble trench foundation or a stem wall or infill for cob walls. A dead tree is a roof support or milled down is siding or a door ect ect. It doesn't end. There are plenty of materials out there that people have deemed garbage. But just because they no longer are usable for there intended purpose doesn't mean they don't have a purpose for something else.
Stay open.