I've made homemade pasta with AP, 00, semolina, and durham flour. Now, I'm interested in experimenting with incorporating farro (because I had a great farro pasta dish at a local restaurant) or rice flour (because I have some gluten free friends). How should this be done? What are the rules?
You should have no trouble making egg pasta with farro. There are a million different recipes for egg pasta, and as I'm sure you've discovered. It's all about developing the gluten and having a good water content. I like to use two cups flour to two whole eggs plus four yolks, a tablespoon of olive oil, and a pinch of salt. The mistakes I made early on were usually around not kneading enough or using too much flour.
Rice flour, on the other hand, is another story. It is a lot harder using a pasta machine to roll out flour that has no gluten, so you may want to roll by hand and use for recipes that are more forgiving of the thickness of the dough.
From the pasta grannies, it looks to be the same as regular flour; most recipes I've seen, add a little bit of regular flour.
(anecdotal) I've taken a pasta class a few months ago in Rome and we used farro flour and the ratios were more or less the same as regular flour.