oh the wonderful wealth of info that is pet store employees....
sorry to say but what they told you is quite wrong, a cycle can take anywhere from 2 to 5 or 6 weeks (depending on the size of the tank and unknown factors) and should be done fishless. IMO there is no way to get through the cycle with fish without causing stress to them, as they really really need 0ppm
ammonia, 0ppm
nitrite, 5-20
PPM nitrate. you can help the cycle along with tetra safe start, and by adding food to create ammonia as it breaks down, but please no fish