There’s nothing on whether they feed hay in the field. Can OP separate the two thin TB’s from the fatties and give them a small paddock with more hay? I like the cubes in a trough idea with alfalfa cubes soaked as well- I’ve never tried it personally but knew a 34 year old horse that thrived on it and looked half her age. Alfalfa is a great stomach soother as well and a round of ulcer treatment would not go amiss.
I’ve never had problems putting weight on a horse with rice bran and alfalfa pellets, but I also fed them 2-3 times a day. 
