If you still had the same mix of tangs now that you had a "few years ago", you might be able to make a judgement as to the appropriateness of that stocking level. Problems do not always happen overnight, they develop as the tangs mature.
As to the original poster, I would not add the purple tang with another zebrasoma sp. I certainly would not add a Naso.
Regarding the Achilles, you might read the primer for Achilles to see what experiences others have had.