The recalbox root user password is recalboxroot. Change the password as described here.
Batocera root user password is linux. Change it as described here.
Install needed software:
sudo apt install mame-tools
Then you can convert/compress PS2 ISO files to CHD:
chdman createcd -i in-filename.iso -o out-filename.chd
CHD files can also converted back to ISO files.