IMHO, it depends on several factors, the first being the experience level of the aquarist. If you are just starting out, then I would suggest allowing your tank to mature a little, and in the meantime, get to know the in's and out's of successful marine aquarium keeping. If you have been keeping marine tanks for years, then you already (should) know what to look for and how to take care of problems that will arise.
How you set your tank up plays a big part as well. If you started with dry rocks and sand, then allowing the beneficial bacteria and larger lifeforms to establish themselves will go a long way in determining your long-term success. If you are using already established live rock then you have already gotten past a large hurdle. When I started my current 120DT, I used mostly dry rock and sand, but added several choice pieces of rock from one of my other tanks that had been running for some 20+ years and seeded the new, dry sand with a couple cups of sand from the same established tank. There was no perceptible cycle at all, and combined with my previous hard earned experience, I had no qualms about adding SPS frags within the first week.
About what you should start out with - you need to think ahead to what you want the tank to be 5-10 years from now and choose appropriately. Can you start out with "easy" corals like many of the soft corals/mushrooms/zoanthids? Sure, but they can also end up being a major problem if/when you make the switch to more demanding species because most of the "easy" corals are not only more tolerant of less than perfect water and sub optimal lighting, they can spread like weeds and then become difficult to get rid of.
hth!
