The only reason I have seen for use of 32-bit MPI is for compatibility with old 32-bit installations. If you have evidence that your application benefits from 64-bit compilation, I question why you would spend time evaluating a return to 32-bit. Restricted address space in 32-bit makes quad core CPUs useless for many applications.
I'm not sure what you mean by your question. A given copy of mpif90 would be normally have a specific compiler built into it.
For example, in Intel MPI, bin64/mpif90 invokes gfortran 64-bit, while bin64/mpiifort is identical except for invoking ifort 64-bit. A range of versions of the Fortran compiler will work with the same MPI script.
If you built openmpi yourself, on linux x86_64, you could choose any Fortran compiler, 32- or 64-bit, and would use the same compiler (possibly a range of versions, but not switching between 32- and 64-bit) for building applications. I assume a 64-bit openmpi is most likely to be present when provided by a system builder, as I've never seen anyone use 32-bit openmpi.
