Adding PS6110 v9.0.2 to a group with a sole member running 8.1.3

I have an existing PS6210 in production running PS Firmware 8.1.3. It is the sole member of the group, eqlgroup. We were able to acquire a PS6100, I spun it up and made it a member of its own group, eqltestgroup.

The PS6110 is running PS Firmware 8.1.4 and I want to update it to 9.0.2 before adding it to the production group, eqlgroup (to save myself the task of updating the thing with production data on it).

Can I upgrade PS6110's firmware to 9.0.2 and then add it to the production group? Will eqlgroup accept the PS6110 after the update? Or do I have to update the PS6210 to 8.1.4 (if I don't update the PS6110) or 9.0.2 (if I update the PS6110 to 9.0.2)?

Honestly, I really want to test this myself but I don't want to do these kind of things on production stuff. Your advice is greatly appreciated,