steam works out the box...MATE,cinnamon, KDE, Xfce,LXDE,GNOME ect ect are all desktop environments the beauty of linux is you can install and uninstall all of those desktop envitoments at any time you what (its even easier to do that on mint becase you can just do it from the app store thats pree installed)
So if you installed xface and dident like it you can in a cuple of minutes install a differnt one.
Or you can install ALL of them and pick a desktop enviroment you what to login in to at the log on screen and use a differnt one every day if you whanted to
Quickest and easiest way to create a persistent Linux Mint install on USB (one where you install/customize something, and it's still there after reboot) is to create a VM-->boot linux mint live CD from ISO-->connect USB to VM--> select the USB drive for Mint install.
Just make sure you install your bootloader to your USB and the drive root /dev/sdX not partition /dev/sdX1. Eject / disconnect from the live environment/VM and you're done.