The only vitamins/electrolytes we give our chickens is made by Durvet. It is a powder that goes in their water.
We give it to our show birds prior to a show, to help reduce stress. We also give it to the chickens who are sick and are on antibiotics. Since we live in Tucson, it gets hot, so I will give it to them for the heat, also.
The dosage is 1/4 tsp/gal of water. You give it to them for 5 days, off for 5 days, on again for 5 days, then off for 3 weeks. I got this information from the manufacturer after I bought it, because there was only dosage for swine and cattle.
As for meds to have on hand, I have a broad spectrum antibiotic, Terramycin powder, 1/2 tsp/gal of water. I also have an injectible Tylan 50, .25cc for bantams, .5 cc for standards, for respiratory infections.
I also have VetRx on hand for stuffy nares. I also put on combs, wattles and legs, before a show. Showman/Breeders use it to help prevent illness in their birds, when exposed to several birds. They give a couple drops in the beak a couple of days before a show.
Hope this helps!