I’m assuming it’s a dry yeast? If so they are packaged with nutrients to help kick start fermentation. Making a starter depletes the yeast of these nutrients. If you want to up you yeast count pitch another package. By the time you figure the cost of the fast pitch you’ll have the same amount of cost in another package.
With that I wouldn’t use a barley based starter for a mead. Especially if you’re serving to gluten free individuals.