I think that both those species are too big for your 125. Unicorns get HUGE, like 2 feel long and sohals still get over a foot long. That being said, my first advice is to use a QT tank before introducing any new fish and especially a tang. They are notorious ich magnets and moving them can be enough stress to make it show up. Once you have QT'd the new fish for a month then you can introduce it. The rule of thumb is to get a tang with a different shape and you are less likely to have problems. Rearranging the aquascape can also help reduce aggression.
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