Horn is as hard as horn, if you see what I mean. If a shire horse has good feet the horn will be as hard as the good horn on a tiny pony. Just because they are small does not make them vunerable.
I trimmed a set of feet that were one and a half inches across the other day...now that's tiny.
Our Mini Shetland wears shoes in the summer when she does a lot of road work, not in the winter though. The only reason we decided to shoe her was because she became foot sore on the road. She is so happy in her tiny shoes, (I'm not because they are a devil to put on, Shetland ponies' feet are too close to the ground!!) Her shoes are 2¾ inches. I'm going to get the local jeweller to make the next lot!!
If the horse is happy, sound and confident on grit and stones and the road then don't bother with shoes. If you are doing a lot of road work or on sharp gravel and she goes pottery then get some shoes on.