The supplements all come individually. Most (such as vitamin E/C/B) provide human doses on the bottle so purchasing capsules or liquid and using fractions is best. Also, dosing recommendations can be found online. The Homeopet/Tasha's Herbs are both made for cats.
Regarding the essential oils, they CAN be toxic to cats, yes. However, we have treated literally HUNDREDS of cats and kittens in this fashion under the guidance of a holistic veterinarian (with the really ill ones we have even applied minute amounts of oregano oil directly to cats noses - although I do NOT advise this without PROPER supervision) and we have NEVER had a reaction.
It is my experience that the reaction level of cats to essential oils is overblown much in the same way that chocolate toxicity is in dogs. Obviously it's not completely equatable, as cats are much more sensitive than dogs, but a few drops of the oil in several litres of water in a vaporizer are not even remotely enough to injure a kitten, let alone a full grown cat.
Without the use of these oils and a vaporizer, it is my opinion that we would have lost many of the kittens/cats we were able to save.
Oh, and I highly doubt that the humane society tested them for FIV - but I would double check to be sure. No, it would not have been included in pre-an blood work.
BTW - the link to littlebigcats was referring to the process of diffusing oils, which is placing essential oils in a full strength solution above a heat source. They also recommended when using directly to dilute at least 10-1. I'm suggesting a dilution of THOUSANDS to one, and then diffusing.