I now have a C-4 and a C-7 hub operational (thank you @BobbyD). I am starting to experiment with Hub Mesh and have some devices now on the C-7. I understand the release levels must be consistent between meshed hubs.
Is there a best practice for updating two hubs that are meshed? I don't want to mess up my mesh.
There have only been a couple updates (and one of them might have just been beta...) where it was imperative that hub firmware versions matched with Hub Mesh due to changes in how it worked. So while I'd say it's still a good idea, it's usually not something I'd be concerned about in the short term unless noted otherwise.
Personally, I just update all of my hubs at around the same time. Due to factors like varying numbers of devices, database size, and hub model, some hubs come back up quicker than others. I think the biggest concern is just what events you might miss while one (or more) hub is down, like when it's rebooting for this, since it goes without saying that a hub that is not online cannot send or receive an event to/from other hubs. This may matter for some of your automations (e.g., for motion lighting, I know if I leave a room when I'm starting an update that the lights might not turn off my hub starts rebooting before then--so I'll check later).
This is about all I keep in mind, unless, again, otherwise noted. This would be in the release notes. If there's a better way, I'd like to hear it. 
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