Forums are an excellent resource to help give you advice. 
I think you meant "add the MOST aggressive ones last"?it is really hard as a newcomer to hear very different viewpoints on many things. As stated above, it is really important that you do your own research prior to purchase of new livestock......you can listen to the advice of your LFS but remember, they are ultimately in a business and make money by selling things. Fish of certain species can have a general temperament as a whole but there will always be differences in individuals as well as differences based on each individual set up. Damsels are usually pretty aggressive but yellow tails can be one of the more peaceful. The order that you add the fish can also contribute to their behavior - in that if you add a fish first, it is more likely to establish a territory and if it's a territorial type of fish most likely will get aggressive toward new inhabitants getting in their territory. If you add a more territorial fish last, you can lessen that tendency. Most damsels can be aggressive and territorial but there are some that are known to be less so. To give yourself the best chance of success, you can add the less aggressive types last. The problem with these fish and other hardy fish is that people add them as among the first inhabitants. Another thing to keep in mind is that clowns are also damsels with the same potential behavior tendencies.
once you do your research as you've been doing and read various opinions, you'll need to make the most informed decision you can. You'll find that to be true on many variables in this hobby
IME, yellow tails are more peaceful than many damsels. I can definitely tell you to stay away from dominoes and black and white striped.
