Derma-IonX is safe to give along with other medications. However, be watchful of blood sugar levels just in case you should need to adjust the insulin dosage. For general good health I suggest owners give a few other supplements and this would be especially beneficial since your cat is diabetic. I like to give digestive enzymes with each meal, and a probiotic such as Purina's Forti Flora and a little fish oil supplement daily.
Be sure to avoid dry food; diabetic cats do much better with canned food which is approved for diabetes. I hope these suggestions improve your cat's skin and coat. Please let us know how your cat does and if we can help answer any other questions.