You can enable root access by typing sudo passwd root and then typing your new root password and then sign into your ftp as root with your new password. Or you can access
/etc/emulationstation/es_systems.cfg by using sudo nano /etc/emulationstation/es_systems.cfg
Psx games typically will run about 1/2 GB a game. (At least as far as the games I have) mine are all in bin cue format but others also work. I personally keep my psx gaming on my PC due to the large ROM size.