My settings for a FAT32 USB having several distros, for testing, fun, rescue files, fix computer problems...
USB should have an ext3 partition named persistence for use persistence in Kali. This partition should contain a file named persistence.conf with the content / union.
You could have persistence for other distros too with other partitions, but I'm only interested in having kali tools up-to-date.
For System Rescue CD, modify boot/grub/grub.cfg and in System Rescue entry, change img_label=MY_USB with the label of your FAT32 partition holding the isos
grub.cfg can be watched in this gist, I don't know which one will be more updated.