Mkay, tried it out and Viberi rebels to the rebel faction, Helvetti Rebels, no CTD. We just have to make sure Iberia doesn't attack in the two turns it takes to rebel on Very High tax setting by keeping the legion nearby or something. The border afterwards on the camp map looks pretty good, and Viberi was a bit out of the way I suppose. Long as Ibs don't attack, it's all good.