We currently have an Unleashed network on 200.7. However, R500's (end of support: 2024) do not support 200.8 or later. Adding a 200.8 device to the network caused severe issues (R500's on 200.7 started rebooting).
We have quite a few, and they are not old devices, so we do not feel it is warranted to replace these yet.
I would like to know if ZoneDirector supports APs on different firmware versions. Reasoning: I assume that R500 firmware will stop being updated at some point and would not want these to become paperweights.
Just to reiterate Syamantak's comments: SmartZone can absolutely support different firmware types. It is one of the platform's main features for customer investment protection.
The principle being that the controller software can be updated to support the latest generation APs and individual Zones configured on the controller can have older firmware in order to support legacy model APs and newer software to support new types of AP e.g. Wi-Fi 6.