Messages in iCloud on some devices, mixed with old devices?
That is exactly how it works. The devices that have Messages in iCloud enabled will sync amongst themselves, while older devices will keep working as they always did.

More precisely:


iOS devices with Messages in iCloud will no longer backup messages as part of iCloud Backup. This means that their backups should be smaller.
Devices that support Messages in iCloud will merge their messages with one another via iCloud, filling any “gaps” they may have had. Unsupported devices will neither merge their messages nor fill in any gaps they had.
If you delete a message/conversation on a device that supports Messages in iCloud, it will be deleted from all devices that support Messages in iCloud. That message/conversation will remain on unsupported devices.
Unsupported devices connected to your iCloud account will continue to receive iMessages just like before. There is no need for Messages in iCloud there.
Unsupported devices setup for SMS Forwarding will continue to receive and be able to send SMS/MMS messages like normal. No need for Messages in iCloud there either.