Principle 1 - Old School
If you can reliably boot System Rescue CD and it shows you an initial light blue menu, you are booting the CD using the BIOS, and it's likely that you can thus boot Funtoo Linux using the BIOS. So, go old-school and use BIOS booting, unless you have some reason to use UEFI, such as having a >2.2TB system disk. In that case, see Principle 2, as your system may also support UEFI booting.