Can I apply a steroid on my scalp to treat redness and itchiness post op?

Yes you can use the topical steroid for a week and see if the inflamation goes down. Take the antibiotic as well. Then reevaluate your situation.

You really need to speak with your surgeon to determine the best post op care in a case such as yours.  Steroids can reduce hair regrowth when applied topically but sometimes there is a role for that therapy.  Sometimes an antibiotic, sometimes believe it or not Benadryl orally can reduce redness post transplant.  

You can use the topical steroid as your dermatologist suggests. It doesn't damage your transplanted hair.