I agree with Hara. I don't see anything wrong other than they sound a bit crowded and mixing that many tangs is bound to cause issues.
I would assume cause of death was stress related. As for who to move, I would move the sailfin and the blue to the 100g since they both are gonna get to large to stay where they are. The yellow would do well in the 80g. JMO
though. Ultimately, the decision is yours. The way it was explained to me is......since the hippo is different in body shape, normally you can mix them with the "rounder" tangs like the yellows and sailfins. But, the sailfins, yellows, purples, etc being all the same shape will fight amongst themselves.