If the image has only a Mega Gardevoir and not a Gardevoir in it, should the Mega Gardevoir tag be the only tag for it and not Gardevoir?

https://e621.net/post_tag_history/index?post_id=897771

The Gardevoir tag was added and removed five times and it was added to other images with only a Mega Gardevoir in them as well.

https://e621.net/post_tag_history/index?post_id=887959
https://e621.net/post_tag_history/index?post_id=862187

Only tag the visible form, as explained in the mega_evolution wiki.

Sometimes new users overtag those, but I don't generally bother reporting them for it. Since those are pretty easy to clean with tag scripting and they usually catch on and stop doing it after a while.