I have to respectfully disagree to with Florida Joe on this one.
DO NOT PUT A COCKTAIL SHRIMP IN THIS TANK! You already have a fish in there, if you create a cycle, which I believe happened already, you could and more than likely WILL kill the fish.
If the fish has been in there for a week and you haven't noticed any spikes, you're not going to, your tank is cycled. That being said, you need to continue to monitor your parameters and take things slow.
I have started numerous tanks by adding cured live rock and live sand and been able to add fish and CUC overnight. I did this most recently with my 155G. I've started ALL of my tanks this way and have started other peoples tanks this way as well. It's basically the norm here on LI to use cured LR and live sand and start your tank immediately. The only reason I even waited the 18hours before adding anything to my tanks is just that the sand makes the water cloudy. I've never had a nitrite/ammonia/nitrate spike. I believe this is what has happened in your tank.
This being said, I wouldn't recommend this to a new hobbyist as the route to take, but you already have, so here we are.
You will be fine, just take things slow because your filtration needs to start growing beneficial bacteria which will help out the tank. You may also want to consider running carbon media.