Firmware Support for FortiSwitch-124G-FPOE

Hi,

We have recently taken delivery of a number of FortiSwitch-124G-FPOE, which is to provide the switching platform for new Wi-Fi points that we are deploying (231Gs).   We run on the majority of our Fortigates 7.4.8 Firmware as this is now the recommended version for our models (mix of 60 and 80Fs).  

When I come to add the switch to the Gate via the GUI, either locally on the firewall or via FortiManager I am presented with an error to say its an invalid serial number.  I have tried several different serial numbers and they all provide the same error.

Support are saying I need to have the gates on 7.6 firmware to support these switches which I cannot find any reference of and I am not keen to run that firmware branch as it also means we will have to migrate off SSL VPNs for some of the gates. 

Has anyone else got these switches running on 7.4.x firmware and if so how did you do it ? Just connect the switch up and manually authorise it via the gate?

Thanks in advance.