Description of problem: I think there should be an option to choose the default desktop during installation of Fedora. Many people would like to make their default desktop other than GNOME.
The package selection screen has checkboxes for all the desktop environments present in the spin you're installing from and are in the comps file. If your preferred desktop environment meets these two criteria, you will easily be able to deselect GNOME and select the one of your preference. So there's really nothing to do here in anaconda.