Yeah, they're not common, however if you want a few good Cassandra and Methos stories:

Days Forsaken by jacquez h. valentine
www.dementia.org/~jacquez/writing/hlfic/days_forsaken.html
is by far my favorite.

Quid Pro Quo? by Kai
http://www.seventh-dimension.org/stories/Kai/QuidProQuo.txt
is hilarious.

Once out of Nature by Selena
http://www.seventh-dimension.org/stories/Selena/OnceOutofNature.txt
is also good and a longer story with a more developed plot and character interaction