I think you are right, I asked for their advice and it seems to be all wrong. Ive learned so much so fast here.If the same LFS sold you this tank and rock and coral at the same time and said you were good to go, they lied and just want your money. Take no more advice from them and don't expect to be able to return any livestock, they aren't obligated and probably not inclined.
Go purchase a bottle of Bio-Spira or Fritzyme TurboStart and use it as directed.
The coral will likely be a loss and lesson learned. You need to learn how to care for coral before jumping in.
As mentioned, a 5G can be tough because of how quickly things can change in smaller volumes, but if you keep up with a strict routine and stick to easy corals, you can be successful.