I am from the camp of treating each horse individually; however saying that keeping it as simple as possible.
n the 40 some years I have had horses, funny - I have never needed to feed any grains. If I need energy (which is just about never) I would add or increase alfalfa hay.
I feel good fats are okay like flax seed. I use a stabilized flax seed I really like. A small amount of stabilized rice bran and a quality vitamin with grass hay (Timothy or Orchard) is good. Adding in alfalfa as needed. If you have horses that have been diagnosed with a particular issue then you can supplement them working with your veterinarian. I had some horses via blood work Vit D deficient. I am too (strange living in California) and I supplement all of us with prescribed Vit D.
There are good nutritionists you can work with as well as your vet - just make sure they are not doing it to promote a product line.