Dont worry that you have done the wrong thing most fish shops I have spoke to do not suggest using fishless cycling, but there is advice here on making cycling with fish less of a problem for the fish
Monitor the water for ammonia, try to do water changes, say 20% regularly (daily, every two days etc depending on the readings (look at the scale with the test kit if its really high, change more often), this will mean the cycling can take longer but less stressful on the fish during this time, ask your fish shop about cycling products to use
with the fish, some add bacteria needed to shorter the time needed to cycle.
Once the ammonia goes down slowly start testing for nitrite as well, the ammonia should hit 0 shortly after the nitrite goes up and you will not have to test for it again, although a few days after you add more fish it would be a good idea to do a quick test again because the bacteria you have is enough for the fish you have at the end of the cycle but a mini cycle can start again when more fish cause more waste, again if this happens do a water change again until it goes down again.
Carry on with the testing for nitrites (I say again NitrItes) and do frequent little water changes when they get high until they down to 0 and you are cycled
Then you need only change the water every week or two, I find using a NitrAte tester kit when you first have the tank helped me work out how often a change is necessary
But this fish should be fine in there if you make it easier on them, and if the unfortunate happens dont blame yourself, everyone here as likely dont the same or made other mistakes from taking fish shop advice, if not then it was surely luck rather than judgement
