Biotin is something I would recommend for healthy hooves, works brilliantly
Horse hoof rescue +Se by carol layton has produced many success stories. Give her a search on the internet! Or connect with her on Facebook. She also designs diets for horses. She might be able to answer your questions
To obtain weight gain, it is important to enrich the food ration in proteins and fats, in order to build muscle mass and fat mass of the, but also in micronutriments (minerals, oligos, vitamins) who participate in the synthesis of these tissues.
Concerning the hardening of it is also recommended to enrich the food ration in micronutriments and especially in biotin.
Have a great Day,
The Destrier Team.