You could probably do it quickly with the tag mapper. I use it as a sort of text-expansion macro quite often.
Something like tag them all "galmag" as you add them (you can do that automatically, just change the "tags to add on newly imported books" while you're working), and then have the tag mapper replace "galmag" with "Fiction, General, Science Fiction, Short Fiction, Galaxy Magazine"
If you add the tag mapper button to a toolbar, then simply opening it and hitting OK will apply all the mapping rules to currently visible books.
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