Please tell me who knows. When i will add SZ#2 with system version e.g. 184.108.40.206.296 into a cluster where SZ#1 have version e.g. 220.127.116.11.163 WHAT will be happening with both of them? SZ#1 will be updated by SZ#2? Will the APs be rebooted & updated by cluster after it? What really will be happenihg?
The answer to your question is that you cannot form a cluster with different firmware versions. If one SZ is on an older firmware then its better upgrade it to the version currently running on second SZ and then you will be able to form a cluster.
One of the reasons behind not being able to form a cluster with different versions is that different releases have different features so its technically not possible to form a cluster.