Yes, you can use the Package Manager with older versions. Many of the packages, however, have minimum compiler versions (4.9 when applicable). It depends on whether you plan on using packages that contain Fortran modules. For example, PLPlot will not work, but LAPACK and BLAS will work fine with all versions of Simply Fortran.
The package manager generally does not allow users to install packages that have a minimum compiler version that is insufficient. The problem is that the server doesn't maintain a "previous versions" index, so you wouldn't be able to access files that previously worked with the compiler you have installed.
Depending on the license, we might be able to add MUESLI to the package manager itself. Would that be acceptable?