When to add SPS (easy ones!)

[Chaloupa]

Our new tank has been set up since early February...it is a 5ft 150g with an H&S skimmer, 2x400w MH with T5's.....50g sump....dart pump, we are adding in a Vortech or Tunze for more flow but there is good flow now.....roughly 180lbs of live rock, sand bed.......has a couple of fish; small Naso (2.5inches), Neon goby, Panda Goby, Trimma goby, African Flameback Angel and some easy corals to get the tank stable.....BUT that all being said....back to the original question;

WHEN can I add in SPS corals ? When is the tank considered stable enough to add things like Montipora digi and such? Might seem like a newbie question but in this I am a newbie as I have failed previously.

Thanks in advance for the help!