FWIW, the two are able to play Zynga's "Words with Friends" but that doesn't require the Game Center.
The iOS9 device sends invites to the emil and Game Center username of the iOS10 device, but they are not received.
The iOS 10 device has the Settings > Game Center but does not have an invite option.
The two can txt w/iMessage, but the iOS9 only has "Message".app (the green one, which receives iMessage - blue - data) and doesn't receive iOS10's iMessage invites.
The in-app multiplayer options are limited to Game Center "friends" but does not have the option of inviting.
The email response I got from the people at Quatrian deferred to their "tips" page (which does not address iOS10) and these fairly useless links: