It’s possible I had both Crypto and Velvet. My two dwarf angels died with no outside/skin abnormalities but they couldn’t swim properly and their gills were inflamed. My tusk and majestic angels were covered in a dust that started with spots and the purple tang and fox face also got the spots on them. Took the day off work, bought meds and equipment and right into treatment.The reason I asked - is that its sometimes uncommon to be able to save fish especially 8/11 with velvet - so CONGRATS on that. I wondered if you may have had cryptocaryon instead or in conjunction. My only advice would stay the same - add the old fish first - to make sure there is no recurrence. Beyond that - adding all the new fish at the same time is probably the best idea.
Ok that makes sense to add old fish first, but might hold back the purple tang and add the tangs together after watching the old fish for a couple of weeks.