CentOS-6 32bit can not boot UEFI or secureboot at all, and needs 'Legacy Boot' enabled.
CentOS-6 (x86_64) can boot 'UEFI with secureboot off', but not secureboot.
CentOS-7 (x86_64) can install on secureboot or 'UEFI w/secureboot off', or legacy.
CentOS-7 (Altarch i386) can install on 'UEFI w/secureboot off' or legacy, but no secureboot.