I am working on a mod right now that adds a few NPCs and I don't know how to add unique dialog to one of them without changing ALL of that characters race's dialog.

Filter by the NPCs ID.

http://www.wolflore.net/viewtopic.php?f=22&t=1394 I would link to the Nexus version, but Nexus appears to be currently down. Anyway, you don't need an account to download from Wolflore.