Definitely introduce the flasher first and make sure the dottyback is a bit smaller. They are supposedly less aggressive, but I've seen some mean ones.
Depends upon the species of flasher wrasse, but that is correct for the dottyback. However, I am referring to the current size of the wrasse vs. the dottyback. Make sure you don't put a tiny wrasse with an already established dottyback. You probably won't have an issue with that species, but I don't like to take chances, either.