As soneone who has worked in an aquarium, NEVER assume they have those tangs long term. They may replace them constantly. I know we did. We had about 24 purple tangs in a 3500g tank, and they beat the snot out of each other. It was brutal. But they have the budget to replace them all the time. If you have that budget, by all means, good luck.
There have been two fish in my life that I have been afraid of, and mind you I worked with sharks, piranha, and electric eels.
One was a queen trigger, and the other was a sohal tang that was mean as ANYTHING. The queen was unpredictable....the sohal was completely predictable and it would make a run and slash any time I put my hand in there.
This is my opinion on the 300g. YOu have a lot of tangs, and some real prizes (Atlantic blue, powder blue and clown) as well as one that will really prove the 300g to be small (the unicorn).
I wouldn't add any more, because it would be SOOOOO easy to tip the balance and ruin it all. IMO you are now playing with fire from this point on, it is truly a delicate balance.