There are few guarantees in adding fish and sometimes not having problems. Keep in the back of your mind that even though you waited to add a certain fish at a certain time, it might not work out. In which order to add first look at exactly what you want to add, there is a general rule as far as which is more aggressive. Make sure you quarantine all fish so that way they can be rested and fat before they have to compete for food. Example on my theory I had a Regal Tang that I had to sell because he was beating up my imperator Angel and Purple Tang. Another Regal I have which is larger than the first is docile to a 1" clown trigger and a juvenile blue face angel. Example two if you where to add Richard, Mark and Gary to a room, which would you add first?( Just Joking) Best bet is to add juveniles to your tank, juvenile tangs don't have razors yet, juvenile angels don't have there sharp gill plates either. Good luck on whatever you do and remember there are no guarantees in mother nature.