It is common to develop skin issues after having had fleas. DERMA-IONX can help with the allergy response. Cats can develop an allergy to flea saliva and also acquire tapeworms from the fleas. If you have not already, bring a stool sample to your veterinarian to check if your cat needs to be wormed for tapeworms. I would also give PROBIOTICS and OMEGA 3,6,9 to restore the gut and reduce inflammation.
If your other cats ingest DERMA-IONX it will not hurt them. I prefer to add the homeopathic to canned food given to the intended recipient so that it increases the odds the right cat gets the whole dose. When you add it to the water you don't know how much your cat has gotten and you will also throw some away when you change the water.
Please let us know how you do.