Steven tested the water and it seems to have almost no Ammonia/Nitrites. So he said it should be fine for me to add some invertebrates. Thus I purchased the following:
Colt Coral
Green Mushroom Coral
LPS Brain Coral (came on the same rock than Green Mushroom Coral)
Boxer Shrimp
4 Turbo Snails
4 Blue Legged Hermit Crabs
Back home I decided the aquascaping needed some work. So I changed it to look like this: