I don't think dosage matters with probitotics since they are live microbes that multiply anyway once they take up residence in the chicken's gut. Too little or too much, doesn't matter.
Mix it into their food for one day at the rate of one-half teaspoon per chicken for five days. Following treatment, give them probiotics for several days to restore good microbes.
Yes, it appears they have vent gleet. Besides the Medistatin internally, you can get some miconazole and smooth it on the vent and the tissue below the vent. This will alleviate a lot of the itchng and burning and make them more comfortable. Get it at the pharmacy where the women's hygiene...
Do you have some photos of this? Vent gleet smells bad and often causes redness and irritation on the skin below the vent. Are you sure it's not just poop buildup?
Nystatin in the form of Medistatin is easy to administer to multiple hens by mixing it daily into their food.
You can order it...