Hello Jim!
Well, that's even worse than I thought. I know that I don't have much understanding about coding, but this just floors me.
I thought that the '&&' would add the second part to the first, e.g.
takesplaces=>^0 - Pre Hogwarts.*$=>0 - Pre Hogwarts&&category=>Kid Fic means in words: If you find a takeslace:tag that begins with
0 - Pre Hogwarts then change the tag to exactly
0 - Pre Hogwarts and
additionally add
Kid Fic to category. Obviously that is not what it means.
On the plus side, I now know why half of my tags work and the others don't! Thank you for your thorough explanation, Jim!