It's going to be a little tight with my current Excel one, but I'll just offset the mounting brackets, and it will be fine. I am still waiting for the UK store to stock the deeper units, as the EU store won't ship to the UK.
The UNAS-Pro is still a solid little unit, to be honest. As a basic SMB/NFS share, it's hard to fault.
In terms of the ONT, it depends on your ISP, really. If it's just PPPoE, you can just create a VLAN and then present that same VLAN to the GW. If, for example, you have an ISP which uses VLAN 901, just create a Trunk on the ONT and then an Access port to VLAN on the GW side. Ive tried and tested both methods back when I was using PFsense, and it worked without issues.