Than you for the question.
The version of IBM i OS supported by the iCluster versions is listed in the Rocket iCluster Installation Guide. For instance, the latest iCluster version available is v8.3 with fix pack level 8.3.2 available. The install guide indicates that it requires IBM i 6.1 or higher with the current cumulative IBM PTF level applied.
The possible condition in your example where you have potential for two versions of the IBM i operating system replicating between each other is supported but it should be noted that some challenges do exist. The favored direction of replicating from a lower operating system version to a higher version is the easiest to manage. This is due to the operating system settings that would automatically convert program objects to the higher version upon delivery or upon execution on-the-fly. Replication from higher OS to lower OS versions is not recommended for full application replication support. If iCluster was configured to attempt to replicate program objects from IBM i OS 7.3 to 7.1, all objects that had been converted for use on V7.3 would fail to restore on V7.1. There are some possible workarounds to make the relationship workable however the best solution to work through OS level differences is to upgrade the target system first to the goal version of the OS and then follow that shortly (within the next week or so) with the upgrade of the source or production side of the replication cluster. (OS version must remain in the IBM supported two release differences maximum) That allows for the highest reliability and fewest special conditions that need to be managed.
Should also be noted that each node in an iCluster cluster must be at the same version of iCluster and fix level.