It is difficult to manage everyone's different supplement requests. I have a whole cupboard full of tubs, drops, liquids, etc. Mostly herbal remedies. Any medications are only administered with a vets prescription and instructions. I would not feed a product that I am unfamiliar with unless I spoke with my vet first.
Just a note...as a BO I would never accept a horse in anything but good condition. It is just simply too much work to be responsible for re-conditioning of another persons horse. Not to mention the added expense on your end. You should look at every horse, get references from the previous BO or caretaker and check that the animal has had regular health care issues attended to such as vaccines, worming, hoof care. I also document a health history with the owner and inquire about respiratory problems, past colics, founderings, etc.