2 things here.
1.) If you are showing ammonia but no nitrates or nitrites then you are either at the very beggining of the cycle or something else just very recently started making ammonia in your tank.
Either way at .5 ammonia you are going to go through a cycle and either way its jsut starting. next you will see the ammonia go down and the nitrities go up and then lastly the nitrites will go down and the nitrates will go up.
when your ammonia and nitirites are at 0 and your nitrates ar somewhere around 40 then you do a 10-20% water change and start adding a clean up crew, if the CUC does of for a week then you can think about adding a fish (1 fish).
unfortunatley, it sounds like the new LFS isn't telling the truth either. Sureley they must know as a fish store what a cycle is and how it works and yu are clearly at the very beggining of one.
I know this isn't the news you wanted to hear.
2.) if one fish in your tank has ich, they all have ich to some extent at this point. If a fish is healthy and not stressed it can fight off the external ich that you see but if there is ich in the tank it can still get in thier gills which are basically unprotected by the fishes defense which is a slimecoat on the body.
The little white spots you see are actually the scars from where the ich burrowed intot he fishes flesh, the ich is in there and very safe from any treatments at that point.
After it's fed on the fish for a time it will drop out of the fish and attach itself in a cyst form to something hard in your tank. (Live rock, sand, glass, powerheads, a snails shell, a corals skeleton, anything hard) and it will sit there like that for up to 28 days basically making babies inside this little hard ball. after it finisher making babies it ruptures and these free swimming babies go looking for a fish to feed on. they have about 18 hours to find a fish or they die. Now naturally in a small area like a fish tank they don't have to look very hard to find a fish since the fish have nowhere to really go.