Do the Amish eat things that are considered, in the Old Testament, to be unclean?
Yes, they do. They are quite fond of pork and bacon makes everything better. They will eat dairy in the same meal as meat, and enjoy shellfish when ever they have an opportunity to do so.
They believe Matthew 15:17-18 applies to all food choices. To them food is food, they will thank God for it and ask Him to bless it and then proceed to enjoy it. Meals are simple, but hearty and delicious consisting mostly of foods they have raised themselves. Since they don't raise shellfish they hardly ever eat it other than when they travel and stop at restaurants to eat.