Oh dear; LFS strikes again. Yes, adding fish will add ammonia to the tank. Unfortunately, ammonia will also permanently damage the fishes gills, and may kill some of them. Can you ask the LFS if you can have a little used filter media (sponge) to kick start your own?. As for SafeStart, it was tested in Practical Fishkeeping magazine and although the ammonia didn't get very high, the Nitrites still did, which is almost as bad. The LFS want you to put fish in now? - of course they do; if the ammonia kills them, what will you do then?. Go back to the shop and buy some more, of course. Not all LFS are the same, of course, but this is a very familiar story. Just spend some time reading the pinned threads on this site and you should get a much better picture of what happens when you start a tank. We're not here to take your money, either. Just don't rush things at the start; take the time to get things right now and you'll save a LOT of problems later on.