It's a bit strange how it works, NO physical connection are made and it's just a straight swap, connections wise, from the Sofar to the Sunsynk, albeit with one less CT clamp. The Sunsynk is then set up using the menus and it takes care of itself. I dont yet have a PV system additional to the existing FIT system but intend to at some point in the future, so I'm unsure of how it would be set up but sure that so long as the Sunsynk is set to not export to the grid (using the CT clamp to recognise export) then the additional PV should just charge the battery and power the house. If it helps, during PV production at the moment my FIT system both charges the battery and provides power to the house at the same time. When the battery is full it then sheds any excess to the grid as used to happen when I had purely a PV system.
Just had chance to look at the PDF. If you remove the Sofar me300sp and the non FIT inverter, just run the non FIT PV into the Sunsynk, would that work? It's similar to what I have, minus the additional PV, though I am hoping to add an additional PV string(s) in the future.