IMO, I would make sure your tank has fully cycled and you're keeping water parameters within spec (i.e. pH, Alk, Ca, amonia, nitrite, nitrate). If your water quality is good, I think you should be ok to give the sun coral a try. I have one and it seems quite easy to keep. Just make sure to feed it (feeding is pretty cool to observe the polyps taking down all the food) since it's not photosynthetic & needs food to survive.
Good luck. HTH,
Rob