Hmm...If you have to go with a self defense argument, Romeo could perhaps argue that Tybalt would have killed him eventually if he did not kill Tybalt. This is a HUGE stretch, but it's all I can come up with. Technically, as the previous poster points out, Romeo was not in immediate danger before he himself went after Tybalt. Is there enough of a history between Romeo and Tybalt to suggest that Tybalt was bent on fighting/killing Romeo? Can you prove that Romeo had tried to make peace but without success?
Romeo cannot legitimately argue self-defense for his killing of Tybalt, because Romeo initiated the fight. Romeo was overcome with guilt, desire for revenge, and his sense of honor when he learned that Tybalt had killed Mercutio. Had Romeo not summoned Tybalt to a duel, Tybalt would not have been killed.
A plausible argument that Romeo could plead in his defense, if the trial were taking place under Elizabethan law, is similar to the old American common-law defense of "fighting words." It used to be that a killer could justify a killing by demonstrating that the victim "asked for it," by insulting the killer's family or otherwise bringing dishonor to the killer. This is, in my opinion, the only defense that could be successfully mounted on Romeo's behalf.