Yes, you can use an SSD with the players or NAS RIP 4K, but you will need a 2.5 inch to 3.5 inch SATA HDD SSD Adapter that Supports SATA 3.0.
Zappiti Transfer is an old software dedicated only to the first generation Zappiti Player 4K. You do not need it for the Zappiti NAS RIP 4K HDR transfers or the Zappiti 4K Pro 4K.
For more information about transfers, click the link below:
The WD Red HDD currently works with the Zappiti Pro 4K HDR, Zappiti One SE 4K HDR, Zappiti One 4K HDR, but not the Zappiti Duo 4K HDR.
The limit of simultaneous playback is 6 at the same time for one server. If you want 12 zones at the same time, it would be better to add an additional server for improved performance. The second server can be a Synology with a replication of the Zappiti NAS.