As far as "Science Fiction", "Sci-Fi", "scifi" goes, you may just want to edit those tags directly, since they are the same thing, but not conforming to standards across metadata sources. I would think it's better to have these changes available directly in the tag browser, and of course it will shorten what may become quite the list.
Any tag can be edited in the tag browser and the change will propagate across all books with that tag, merging with a pre-existing tag if necessary.