The cap'n is right, re the ideal qt [quarantine] time.
One other point: it really, really helps if you can plan the community you want, do your stocking of fish once and for all within the first few months, and then plan not to get another fish for the next ten years. OR at least not for a long time.
The fewer fish coming in, the less chance of something obnoxious getting in while the tank is still in its first-year fragility.
You can qt several compatible fish together, which might make it advisable, if you can manage, to get a somewhat larger qt tank---so you can get them all in as fast as possible and end the period of 'exposure' to outside disease. This requires a bit of research on compatibility, but it is at least pretty sure that if they were in qt together they're not going to turn on one another when they hit the tank.
Look at the adult size of the fish in question: some tangs get to be super-large, like pushing 2 feet. Look at the behavior: a grouper or eel will eat anybody that fits in his mouth, and his mouth is wider than his body...etc. And look at the diet: can you supply him without polluting your tank or over-taxing your skimmer? I recommend going to a place like liveaquaria.com, looking at marine fish and reading the text with them, plus looking at the compatibility chart they provide. It's the most educational fish site I know of.