Your best bet now, beyond taking all the fish back, is to do the daily water changes and keep monitoring the ammonia. Your tank will cycle this way, but it's pretty likely you will loose at least the rest of the neons. Maybe the store would take back the neons if you are set on keeping some of the fish in there while it cycles?
I thought I'd say something to this. Heat is correct that in most cases fishy cycling can prove fatal to some fish, and with the ammonia already going high, there may be irreversable damage done. However. It is a common myth IMO that a fishy cycle cannot be done without harming fish. It is also a little foriegn to me to reccomend taking fish back part way through a cycle. I am a firm beleiver that the fishless method is far superior for many reasons. However with that being said, fishy cycling can still be done without harming fish. I did many of them in the old days, as have others. it is up to the hobbyist to work a little harder for a few weeks to keep the fish safe. Lots of testing lots of water changes. keep ammonia and nitrite below 0.5 ppm at all times. this may mean 2-3 water changes a day with a heavy fish load. Fishy cycling doesn't have to harm fish and can be quite effective. in this case I would not reccomend taking any fish back that were exposed to 1.5 ppm ammonia, I would reccomend dilligent water changing and maintenance, and vigilent testing until the cycle is completely estabilished. then if overstocking is the case I would reccomend the fish be sold given away or re-housed after they prove healthy. Even then ammonia does cause permanent damage and whoever receives the fish should know that they went through an ammonia spike. I would not want a fish that had been exposed to high ammonia going back into my tanks quickly if I were an LFS owner, too much potential for problems. It is easy to make mistakes, take poor advice or generally just goof things up when we are learning or any other time for that matter. however once we know the error of our ways it is far better to deal with it and learn from the experience than to give away our problem and start over. Just my opinion, keep doing water changes, keep the levels low, and learn all you can for the next time. we were all new once, and hindsight is alwyas 20/20.
dave