Magic Banana answered the first question concerning the MATE installation. It is quite easy, and instructions can be found at the https://trisquel.info/en/wiki/installing-mate page.
As to the second question, converting to Trisquel Mini, you can't really 'convert' to Trisquel Mini. Rather, Trisquel Mini is an ISO image which installs a prepackaged LXDE desktop environment and a few extras. To install the LXDE desktop environment as used by Trisquel Mini, install the package lxde and log out. A new entry should then appear in the login drop-down allowing you to choose LXDE as your desktop environment to log in to. Good luck.
