1 1/2 gallons morning and evening sounds like quite a bit!
I would start off with about a pound, which is usually just less than a full small coffee can, and they gradually add a little more over time.
What I usually feed my horses as their evening "treats," and they are all very fat and happy, is a small coffee can of grain (usually sweet feed), a small coffee can (or two) of alfalfa pellets, and a drizzle of veggie oil. I put all that in a bucket and pour some water over it at let it soak (only because I am paranoid about not soaking the alfalfa pellets). Anyway, they end up with a really tasty wet mash that they love.
I really have no good suggestions on the vitamins, because I am trying to learn more about that myself. I have never been a big supplement type person, but I do buy the Purina Enrich 12 (or Enrich 32 if you are not feeding mostly alfalfa) and give them a little scoop of that and it is supposed to compliment their regular diet and give them all their vitamins and minerals. But that stuff has really gotten pricey in the last year or so.
I personally wouldn't feed more than about 3 pounds of grain a day total, and if my horse still needed more weight would add more alfalfa to the diet and some type of fat supplement. I think grain makes a great treat, but I would be scared to feed lots of it.