Thank you for all your replies!
I think I'll just sum up everything I understand:
The PPAs one can add to one's sources as described in the documentation are
not controlled by Trisquel or Canonical and therefore are not safe in regard
to freedom.
If one isn't happy with the default desktop environment it would hence be
best to setup a netinstall and install one of the DEs available from the
standard Trisquel repos like XFCE, KDE, LXDE, etc. to save oneself the pain
of having to uninstall the default DE, which is some kind of GNOME flashback.
