So don't rely on the LFS advice at all, if they don't give you accurate advice. Remember that they are in the business to sell you stuff.
The best thing you can do is to research the fish that you want before buying them. Research compatibility, max adult size (that's a big one!), diet, behavior... find out everything you can before you purchase so you won't find yourself in this situation again.
Don't add any more fish until the tank cycles, that will only make things a lot worse. Keep testing the water, and change enough water to keep ammonia and nitrite below .25 ppm. This may mean that you have to change water once a day or more, but 3 ppm ammonia will quickly kill fish (saw it in the other thread) and that is a situation that needs immediate attention. When the tank has cycled, ammonia and nitrite will always be 0 ppm, and nitrate will be detectable (preferably under 20-40). Then you can go to weekly water changes, vs. the daily water changes that you should be doing now. If you haven't done so already, I suggest you take a look at some of the articles on the forum, especially those that pertain to the nitrogen cycle.
