A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Can I interrupt Saigyo duel with Aroru Sensei?
Aroru Sensei Interrupt, w: After another player refuses a challenge or loses a duel from an action that targeted his Personality and your Ninja, give a -1F/-1C Poison token to his Personality.
Shosuro Saigyo Ninja Battle: A target enemy Personality challenges Saigyo. Give the winner +2F.
It would be hilarious if Scorpion ninja sensei would not work with the only Scorpion ninja able to create a duel, but hey...
Can I interrupt Saigyo duel with Aroru Sensei?
Aroru Sensei Interrupt, w: After another player refuses a challenge or loses a duel from an action that targeted his Personality and your Ninja, give a -1F/-1C Poison token to his Personality.
Shosuro Saigyo Ninja Battle: A target enemy Personality challenges Saigyo. Give the winner +2F.
It would be hilarious if Scorpion ninja sensei would not work with the only Scorpion ninja able to create a duel, but hey...
You are correct, Aroru Sensei does not combine with Saigyo, as his ability does not target your ninja.
You are incorrect, however, that Saigyo is the only Scorpion ninja able to create a duel.