The only problem I encountered when changing my tank over to a sand bed years ago was adding it too quickly (I was told). I would go very slowly, pvc works good as suggested. Otherwise you will suffocate the life forms below.
Aside from that, having the mixed bed has never caused me any nitrate problems.
BTW, you wouldn't want to mix it....as in stir things up...
Eventually, the larger pieces come to the top anyway
