I don't see issues with most fish related to properly done formalin baths - the exception would be fish with wounds and maybe baby fish
The only wrasses I ever dipped were 2 sixlines and they handled it well. I have no first hand experience with tangs, but seriously doubt they are more sensitive than all other fish that handle formalin baths well.
As for TTM - I only do it right away if fish arrive with clear ich symptoms, otherwise I rather wait.