Yes you can use both types of raceways in the same run but the RMC is required to be connected to an EGC unless it complies with 250.86Ex(3).
Seems there are two options:. Use concrete to reduce the cover required over the PVC, or use concrete over the Rigid per the exception to 250.86. presumably if you got to use concrete either way, just skip the rigid and run PVC the whole way and use concrete where needed.I believe that you can go shallow and cap it with 4" of concrete.