You are doing great! Your LFS
gave you some good advice, without a good background explanation. I have a web page that explains my "go low, go slow" philosophy. I just havent been around in a while to promote it. Check out
I think that you can add some bioload in a tank that size. The trick is to go very slow. Imagine your bacteria growing on levels of ammonia well below your test kits lower limit. Yes, they do grow and thrive at less than 0.25ppm of ammonia, after all, once the cycle is done that is situation normal! Your goal is to have the fish bioload adding ammonia to the tank so slowly that bacterial growth can keep up, and keep the ammonia below 0.25ppm.
Take a look at my web page. You will probably want to add a few inches of fish, then wait a week or two. Then a few inches more. Around 45 days or so, if you have not been doing water changes, I bet you start to see your nitrates creep upwards without seeing a spike in ammonia or nitrite.