What they told him was a lie and they are only trying to spend your money on something you dont need.
The reason the ammonia is up now is because the tank is cycling, the bacteria is feeding off of the dead bacteria and will multiply. Until this happens the tanks levels will fluctuate until the bacteria catches up. Once that happens the levels in the tank will 0 out.
When you have a cycle the ammonia and the nitrite go up, when the cycle ends the nitrite wil become o. The ammonia might not be totally 0 so a small water change will do that.
You never want to add anything living until the cycle is over, otherwise you run the risk of killing anything you ad during the cycle.
After the cycle is over which can take weeks, you will want to add a cleanup crew. Which consists of snails, crabs.