I do like some stories where Cassandra is portrayed badly, but many people just exaggerate it beyond belief. I also dislike the stories where the author tries to make Methos conform to modern-day ethics by saying that he was tortured by Kronos into joining the Horsemen and he's really a kind, gentle person. I'm more for that he joined/started the Horsemen willingly but things changed as time went on, as in your "Square Peg" story.

The idea of magic tags makes sense. I have no idea how the stand-off would be resolved though. I've read a few crossovers with the Dresden Files but not enough to really understand the series.