It's largely up to the transceiver as to what speeds are supported. I assume you are asking about a copper transceiver, as that's the only place I've seen 2.5Gbps.
For me I am unaware of any SFP+ transceivers that support 2.5Gbps. However if you look at the data sheet you can see the supports the 10GBASE-T standard. This is a different standard than 2.5GBASE-T. So I will say almost certainly this will not work.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.