For your STB mare, she looks a little fat (they should be a "lean" horse, like a Thoroughbred). I would feed just the 3 lbs of Alfalfa pellets a day and add a vitamin supplement, like Select II, Grandvite, or Equi-Base Grass (by Uckele). Some kind of broad vitamin that will take the place of the feed pellets. She shouldn't need the extra calories and starch from the feed pellets.
If they all have free access to hay, then feeding their "goodies" just once a day is fine, especially since you're feeding such a small quantity.
They do not need beet pulp. I'd take it back to the feed store. I only use beet pulp as a hay stretcher or for horses that are VERY thin and being rehabilitated. Or for senior horses that can't chew hay very well (missing teeth).
If you want to feed them all the same thing, switch the minis to alfalfa pellets with vitamins once the pelleted feed runs out. Use some water or apple cider vinegar to mix in powdered vitamins if that's what you get.