Questions - Need some input

I am building out my SPS dominated reef. My question is when most people refer to a colony, are they just refering to an sps piece fully grown out?

The second part of that is do all frags grow into colonies? For example i have a nice birds nest that is the exact shape as a half circle and 5 inches in diameter - i assume this is a colony. My staghorn acrapora is nice, but it grows up and sprouts a few branches - but does not look like it will ever become that half circle colony, it ...

i am looking to add some fish. Since i have had prior problems with semi-aggressive fish, i typically stick to only peaceful fish. I am definately getting a heptus tang (spelling) and would also love to add a yellow tang at the same time. the yellow tang does say semi-aggressive especially toward other tangs.

I see these two together in alot of tanks. Does anyone have experience with both or how aggressive a yellow tang really is.
I really dont want to put him in my tank ...