black cans would be up to you, but I'd use it on plat units because a player generally levels more plat units than blacks in the long run, and would thus lack more plat fairies than black fairies. essentially, a black can saves 12 plat/black fairies (because 1 black can = 5 plat armor; you use 3 fairies with 1 black can and 15 fairies for 5 plat armor to achieve the same exp. you can view black can as saving plat armor, but I think in the long run you'd lack more fairies than plat armors due to using 3 fairies per can, and using 2 or more fairies with farm event dupe copies)
Cost reduction should probably be focused on units you tend to deploy early. lecia is actually one of the units you want at low cost, but the fact that she's an event unit kind of makes people not want to use CR fairies for her. Kayou is probably one of the units you might want at a decently low cost, since getting her out fast and having her reduce boss hp by 20% might be nice.
there isn't an up-to-date tier list now i think.