FIrst of all, they just wanted to sell you more fish because that is their business and they want to make money. If there is ANY ammonia in the tank, you should NOT be adding any fish!! Any ammonia at all means your tank is not cycled and the fish won't be healthy until it's finished cycling. If you add more fish, it will make it worse.
The additive may help for now, but it's not a long-term solution. So if you can destress the fish with it, that might be a good idea.
You should get your own test kit so you can check your own water. It'll be easier than constantly bringing it to the store. Takes only 5 minutes. Buy the API Freshwater Master Test Kit, it's the best value for your money (does over 800 tests) and is reliable and accurate, unlike strip tests, which are not able to accurately test ammonia, nitrite, and nitrate, which are the three most essential tests.
If I were you, I'd go back to the store and yell at the guy who sold you more fish when you had ammonia in your water!!! You are stuck in a fish-in cycling situation now, so all you can do is do lots of water changes to reduce the ammonia as much as you can. The test kit will really help so you can tell if you've changed enough water. The cycle will be over in 1-2 months and you will have a cycled tank where any new fish that are added will be much healthier.