Easy tricks that i have tested and proven. Nothing fancy and "storeless"...
1.) Size
You must 1st settle for a much bigger/fiercer new Tang that you want to add, when you have existing tang in your tank.
example.
Existing YT 1", you must select equal or bigger tang when adding. Ie, 1" or Bigger PT. or a fiercer Sohal tang smaller than YT.
This is subjectives to individual character and fitness of each tang.
Size of tank also important.
2.) Territorial
Tangs are territorial. They will vent off any intruders esp similar species.
Personally, i have a much smaller Brown tang that will fight off YT and PT that intruded into its territory.
Hence, before adding new Tangs, be it smaller or gentler, change the territoral scape.
Then add the new member(s).
Take this chance to rescrape!
3.) Health
Health of new member(s) have to be good as well.
Even if its a fiercer species, if it cannot substain, nothing else to say.
Test whether its eating or not at LFS 1st.
Test with the food that you normally feed in your own tank.