Yes, this is supported. This is from 5.4 release notes:
"Note: When using an SSG 500 device and an SSG 500M Series device in an NSRP environment, both devices must be running ScreenOS 5.4r2 or later. Both devices must be one of the following clusters: SSG 520 and SSG 520M NSRP cluster or SSG 550 and SSG 550M NSRP cluster."
Regards,