This variety of fic was particularly prominent during the '90s, as these were the two most prominent Shōjo and Shōnen series at the time; it still remains surprisingly popular.
See Magical Girl AU for fanfics about characters from non-Magical Girl works becoming magical girls/boys; Common Crossover for common crossover fanfic targets; and Intercontinuity Crossover for crossovers between two or more completely different works.
If you want the other kind of Fuku Fic, the one that actually takes place in school and doesn't have anything to do with Ranma or Sailor Moon in some cases, check High School AU.
For listings of Ranma ½ and Sailor Moon fanfics and Fandom Specific Plots, see FanWorks.Sailor Moon, FanWorks.Ranma One Half, FandomSpecificPlot.Ranma One Half, and FandomSpecificPlot.Sailor Moon.