You should plan on lining the stall areas with some extra, replaceable material. The pigmy ponies will kick stall walls to chase flys, relax their legs and to just plain aggrevate you. They wll want to chew thru it, too. Metal out side siding is popular here in Michigan with a lining of plywood or tongue & groove boards. My current barn is concrete block at horse kick height range. One shot and they change their mind.
If a horse kicks thru the metal siding, chances are torn ligaments and blood poisoning will result. When I had boarders, they had lots of tales about horse stall wall materials. Even chain link fence makes a good lining. They will dent the siding but not break thru the wire. Chewing or gumming the wire tapers off after they realize its hopeless. Be sure to secure the end wire strands.
Pay attention to drainage in the stall areas, too. They release a LOT of fluids. And eventually the stall pits will saturate. A gravel drainage bay with a clay cap is how mine is done.
A concrete aisle-way with good lights will make your farrier happy. Make it tall & wide enough to run a haywagon thru and leave room above the stalls for hay storage. Want to impress boarders? A wash stall with warm water gives extra points. A porta potty will keep them out of YOUR house.
Good planning up front will make your life a lot easier. Sorry about my spewing. I've aeen a few of my hay customers wish they had done things differently. A word to the wise ...