Besides the tank size, the answer to the question "in which order can i add multiple tangs" is (of course) it depends. Fish are individuals and although some species are aggressive, individuals of that species may not be...
However, in my experience there are two factors that seems to be most important when it comes to housing multiple fishes;
1. Is the newcomer considered a threat to the others?
2. Is there already established territories within the tank?
The first question is the tricky one. Fish is generally aggressive towards other fish in similar shape and size. I've had clownfish hunting down a dwarfangel while completely ignoring another clown. A quite large sail fin tang bullying a copperband butterfly, but ignoring the clown tang. The one common factor here is that the aggressive fish went for the other fish because of its size and shape, not because it posed a threat in the battle for food, territory etc... The dwarfangel was about the same size while the other clown was less than half the size of the others and the copperband had stripes and were also similar in size, while the clown tang was less than half the size of the sailfin.
If you want to add multiple tangs at different times, make shure they are different sizes.
When addressing the second question, I've also had great success with temporarly (or permanently of course) changing the reefscape when adding single fish. Most recently I added a powder blue tang to my group of purple, sailfin and regal blue (hepatus) without a hitch by moving 3 pieces of rock, effectively breaking upp territories. When the powder blue settled in i restored the reef. This is obviously much easier when you don't have a lot of coral growing everywhere. I believe the best way to do this is probably to add all your fish first, and then complete your desired reefs cape. This approach may take a couple of months, or longer depending on fish availability and cycling the tank, but i belive that taking the time to do just that is by far the best way to house multiple tangs in peace.
And as always, make shure your reef contains enough places to hide for all fish, lots of caves, overhangs and other tight spots to make them find their own place to sleep, hang when they feel threatened etc... this is where tank size comes in to play. Make shure you have the appropriate size tank for your fish, or you'll likely run into aggression and trouble down the road.
Good luck!
//N