Well, the fact that you care is on your side. If you are willing to put in the research, which I think you are, I feel brighter days are ahead for your tank.
I would first purchase a good marine book, like The Conscientious Marine Aquarist
by Robert Fenner. This will give you all the common (and specific as you grow) knowledge you need as well as being a handbook to refer to in times of question or crisis. This will allow you to break yourself free from having to rely on the LFS
for knowledge, as IMO
already they have led
you down the wrong path.
My take on things:
Since we can't see your tank, or its history....this is all generalization based on what you have stated.
My guess is that the clown died from the resulting stress of going through the cycle. Fish should never be used, the cycle involves sharp peaks in both ammonia and nitrites...both which are lethal to fish. Ammonia burns and can literally melt gill filaments while nitrites bind with the hemoglobin in the blood blocking oxygen transer. The fish did not die during the cycle, but my guess is they were stressed and weakened enough that they became susceptible to the ich present in your tank.
The med you used is probably some product that contains the active ingredient Methylene Blue. As the others have stated, it is not safe for inverts and either that or the CBS is most likely the cause for the demise of the cleaner shrimp. The CBS may or may not make it depending on how it was affected. I would get a polyfilter and run it on the tank to remove as much of the med as possible. In the future, fish should be isolated to quarantine tanks for treatment. For that matter, any new addition should be quarantined first as well. It is very simple and cheap to do even in an apartment (simple enough that it makes no sense for anyone NOT to do it!) and will greatly reduce any possibility of having a malady enter the main tank.
For now, I would not be concerned with adding more fish to the tank. I would be more concerned about researching water chemistry and other basic saltwater husbandry and implementing those practices. Your maintenance schedule sounds good as long as you are doing them in the manner the others explained. Once you are comfortable with the happenings in the tank (your tank will let you know by being stable and nice looking), then I would consider purchasing a new fish, quarantining them, and introducing them to your new environment. I would not go over 3 small fish in a 20g so you are actually alread at your max.
The scooter blenny is tough to keep. They require meaty fare, and many feed mostly on the pods
in the tank. Yours is pretty small to be able to provide enough, especially after being nuked by the med. Try offering enriched brine (even alive if you have to) to get it feeding and then move on to enriched mysis, zooplankton, etc.