I'm going to buy a NAS to back-up different pcs (~15) with Apple time-machine but I'm facing different doubth regarding compatibilities, and possible issues. I will include informations regarding chosed devices and approaches, but I hadn't bought anything yet so choices can be changed.

The general approach (single NAS, local storage backupped with TM) is not a question topic, or until it will be clar that what I'm asking is not possible.

I will quote different pages where I get informations from, because apple documentation is not exhaustive.

Selected NAS: QNAP TS-431P2-4G
MaxOS versions: 10.6.8 , 10.13 and newer

1. NAS support time-machine, support Protocols AFP (v3.3) and SMB and has QTS 4.3

2. MacOS > 10.12 support SMB in addition to AFP for time-machine, QTS 4.3 can run SMB for time machine. No mention has been done regarding the possibility to use both SMB and AFP protocols in the same NAS

   Other link with the same information

   Chatting with QNAP technical service I had a reply:

   Service: Non possono essere combinati i vecchi macos con quelli successivi 10.12 sullo steso nas.
   Old macOS cannot be combined in the same NAS with macOS newer than 10.12

   Me: QTS 4.3.6 o precedenti non supportano contemporaneamente SMB3 e AFP? Non posso configurare un utente per SMB3 (macOs > 10.12) e un altro utente per AFP?Neanche creando diverse partizioni?
   Doesn't QTS 4.3.6 or earlier support SMB3 and AFP concurrently? Can't I set up an user for SMB3 (macOS > 10.12) and another user for AFP? What about using different prtitions?

   Service: non abbiamo casistiche come questo .
   Se desidera procedere lo stesso posso sollevare il problema con i developers ma deve essere pronto a mesi di troubleshooting nel caso ci fossero problemi con i backup.
   We don't have experience about that. If you decide to proceed anyway I can update developers regarding the problem, but be ready for months of troubleshooting if there will be problems with backups.

3. AFP can’t share files on Apple File System (APFS). Apple File System (APFS) is the default file system in macOS High Sierra (10.13) for Mac computers with all-flash storage. You can't opt out of the transition to APFS when you upgrade a Mac with all-flash storage to macOS High Sierra. If you need to share files, switch to SMB. If you have network home directories shared via AFP on an APFS volume, update the mount records and user records to use SMB.

   Other link with the same information

   That with point 2 should means that only macOS 10.12 works with both SMB and AFP

Does anyone had experience or informations regarding a successfull way to use the same NAS to store time machine data with two mentioned OS?

The only option, from infos above, is to have a NAS that can handle SMB and AFP concurrently and that was reported as impossible from QNAP technical service (buy maybe other brands can, or he was wrong).

Thanks for help