The first step is to configure MPICH with the --with-flibname option. 
For example, if one of the compilers is g77, use

setenv FC g77
./configure --with-flibname=mpich-g77 ... other options ...

After you build, test, and install this version of MPICH, you can 
configure support for additional Fortran compilers as follows:

1. Change directory to 'src/fortran'
2. Execute

setenv FC pgf77
./configure --with-mpichconfig --with-flibname=mpich-pgf77

make
make install-alt

To use a particular Fortran compiler, either select it on the mpif77 
command line with the -config=name option (e.g., -config=pgf77) 
or by selecting a particular mpif77 command (e.g., mpif77-pgf77).