Aurigas/Threadfins are one of the more, if not the most aggressive butterflies, but it sounds like you have one that's particularly nasty (the size difference probaby isn't helping either). How big is the tank? Was the Threadfin added before the Pearlscale? So far, my Threadfin is OK with my Racoon, but the latter is no push-over and they were added together (in a 180).
In a butterfly tank I would add the Threadfin last. The flip side to this is that many butterflies are too passive/shy to do well in a community tank that contains aggressive Tangs, Angels, etc. but Threadfins can usually hold their own against other fish.