I am sorry you lost the clown. It is hard to know if your chromis will succumb. The life cycle of the parasite is such that your fish will go through periods of less or more parasites on the body depending on the level of infestation of your tank. Humblefish is the expert extraordinaire around here. His articles and expertise are invaluable and should be heeded. With the level of infestation observed on your clown, time is of the essence. At a minimum, all fish need to be removed and tank left fishless for 76 days. This gives time for the parasite to die off in the main tank. Then fish can be added back without fear that they will be re-infected.
Treatment in the quarantine tank is important, but you must choose a procedure that you feel like you can accomplish and do not mix treatments. Set up a log and keep details of your treatment. I use cupramine and test daily with a copper test kit to make sure you are keeping it at optimal levels without going too strong and hurting the fish. The biggest issue with a quarantine tank is keeping it free of Ammonia. I do big water changes twice a week (50%), but that is me. The means adding back copper to the right concentration each time. With the cupramine, I found that following the directions on the bottle was extremely accurate and allowed me to get the water change water of a known amount ( I did 5 gallons each time on my 10 gallon QT) was easier that guessing . Always test ( I use salifert). There are so many great articles on this site to assist you in being successful with your quarantine process. Keep your next 5 gallons of new water going immediately when you do a change for emergencies. If fish all of a sudden look bad, do a water change without the copper to see if they improve. I feed heavy and change lots of water, to assist the fish in this very stressful time.
Good luck