There have been some limited cases in the past where mixing appliance types in the same ClusterXL setup was supported, such as a UTM-1 2050 and UTM-1 2070:
sk36863: Limitations of UTM-1 cluster that consists of different UTM-1 appliance models
You might be able to make a cluster work with different models, but as Danny says doing so will most definitely not be supported. If ClusterXL can discern any substantial difference between the configuration of the two members, the non-active member will enter the dreaded "Ready" state instead of "standby".
--
My book "Max Power: Check Point Firewall Performance Optimization"
now available via http://maxpowerfirewalls.com.
Gaia 4.18 (R82) Immersion Tips, Tricks, & Best Practices
Self-Guided Video Series Coming Soon