System Coupling, Forte, and Fluent are independent in terms of the MPI choice. If you want to use Intel MPI for all three, you need to specify them all independently. Both Fluent and System Coupling default to Intel MPI.

You can override System Coupling MPI using the command line argument when starting System Coupling:

<ansys install>/SystemCoupling/bin/systemcoupling --mpi=intel

For Fluent, you need to set a similar command line argument, but this time you set it in Fluent participant execution control in the System Coupling datamodel. So, for example, if you have a participant 'FLUENT-1', you can set Intel MPI option by modifying the System Coupling (run.py) script:

DatamodelRoot().CouplingParticipant['FLUENT-1'].ExecutionControl.AdditionalArguments = '-mpi=intel'