Just the brewers yeast is sufficient, and a teaspoon or so daily sprinkled over their feed is good.
Your giving them extra brewers yeast won't harm them, as the niacin content in the yeast is not concentrated enough to cause overdose.
I would NOT recommend adding niacin to their water if you...
Brewers yeast / nutritional yeast can also be found at WalMart, CVS, Wegmans, Kroger's, etc....typically in the vitamin and/or health food/supplement section...