I think you will be surprised by how big and robust the Amano shrimp are if you haven't seen any yet. I know I was. THEY ARE BIG, as shrimp go anyway. You might be good with the GBRs and Amanos. Not to say that this is the norm but I have successfully kept GBRs with significantly smaller ghost shrimp. But ghost shrimp IMO do not compare to the algae busting power of Amanos.
+1 on speedie's advice with GBRs. Though some have reported success with lots of vertical elements to break up sight-lines and for the establishment of territories. I have not had success with more than one pair and I have had GBRs in my tanks on several occasions.
Great choice for a schooling fish. Rummies will make a very nice school in this tank. Consider me subscribed!