+1 CVC
Civic, I'm hoping it was a typo, but it is good to clarify - since you've had copper in your QT tank you can *never* use that tank for anything else - copper can leach under the silicone and will kill any future live rock or inverts that go into that tank.
Also part of the reason you need to give so much time between additions is that the bacteria that process all of the ammonia and nitrites take several weeks to reproduce and catch up to the increased bioload added by a single fish. Fish also need time to establish territory and settle into a tank - adding too many at once can create problems as far as bullying and territorial establishment unless the fish are known to be schooling fish (which most of yours are not). In the chiclid tank you had all chiclids - but here you are mixing many different types of fish who have all sorts of different needs for space, feeding, territory, aggressiveness/passiveness, etc. And one more thing to clarify - size does matter. Just because you remove a 2" firefish, you can't assume that replacing it with a 6" tang will keep your bioload the same. If you had all small fish, then you could probably have a few more fish (as long as you waited the requisite amout of time before additions), but if you had all large fish then you would actually need to keep fewer than 12. Also, once you tank is established (think of this as being set up and stable for well over a year) then you can think about adding 1-2 more beyond the 13 fish limit. I know you say everything is going great - and it probably is right now - but in this hobby nothing good is ever accomplished quickly. Please learn to be patient for the sake of your fish or you are going to have another tank crash. Also, if you add more than 1 tang, you need to make sure they are all from different genuses - the purple and yellow tangs are from the same genus (zebrasoma) and will fight.