Triskel is (as far as I know) simply an option when using the net install
which installs all KDE packages - this can also be achieved by installing a
basic system using the net install and then installing the KDE packages once
you are done. It is not an official Trisquel version unlike LXDE or MATE.
trisquel.info/en/wiki/installing-kde
You can simply install a basic system and then the Xfce packages to achieve
what you desire (i.e. a simple version of Trisquel which only contains Xfce
without unneeded packages).