Like you, I've seen dozens of people on both sides of the fence rearding mice vs rats. There are some limited studies showing protein differences and growth potential and rats seem to edge out mice.

WmHrbst makes an excellent point! The feeder's nutrition is your snake's nutrition. With that in mind I started a small rat colony (1.2) and feed them Mazuri Rodent Breeder. Litter growth is phenomenal when compared to pet store foods containing seeds and grain, which for me translates to better nutrition for the pups. Since feeding them to my BPs they have become noticably larger versus store bought f/t feeders.

I haven't heard of vitamin supplements for snakes, aside from some water additives to help with egg production. Everything they need should come from the feeders. Practice good husbandry and you will have happy, strong snakes for many years.