Well, ultimately, I would say ask your vet. I feed Beet Pulp to my horses. It must be soaked for about 2 hours before you give it because if given dry could expand in their GI tract and cause impaction. I add it as an extra feeding, about 1/4 quart beet pulp with 1/4 quart alfalfa pellets (also soaked). Are you feeding alfalfa? That also is a great, healthy weight builder (providing you can find it without blister beetles). Often the feed store will have pelleted alfalfa, which I have found is the greatest stuff.
What a WONDERFUL thing you are doing - rescuing horses is a true blessing. People who save animals are part angel!