Did the pedi say what kind of rash it is? Is it yeast?
For us w/ a yeast rash the nystatin helped, but so did the OTC lotramin. Lots of air time, baking soda in the bath. We would do a layer of the lotramin, then a thick layer of a desitin type cream to act as a barrier.
No disposable wipes. Just a wet washcloth when needed - definitely not at every change.
My other suggestion is to try more than just 2 kinds of diapers. There are plenty of other brands as well as cloth. We exclusively use cloth and the first diaper rash DD got was at age 1. Some babies have severe sensitivities to the disposable ones.