In my opinion, I would recommend that you buy caplets that are specifically intended for dogs of different sizes. There are sites online (such as Foster & Smith) that provide such capsules that have the intended amount of soy oil for each dog.
The problem with soy products is that they can lead to a disastrous gastrointestinal disorder called bloat. It is potentially fatal for poochies. But the benefits, for the skin and coat as you said, are wonderful when soy is given in the proper dosage. Also be careful to check how much soy is in your dog food. If it is one of the first six ingredients listed, you may want to reconsider the food or talk to a vet.
With that said, it is hard to recommend an amount of soy oil for each dog. Scientists have formulated the soy capsules (which may say Linoleic acid, the same thing as soy oil) to a weight range specific to the size of the dosage. You can buy capsules for your very small dog, and for your large dog as well. If your doggies are bad with pills try getting pill pockets. They are little treat looking morsels that are hollow so you can insert the pill right in and your dogs will gobble it up without a second thought.
