The ports on the SG550XG-24F are all 10Mbps/1Gbps/10Gbps; I suspect the same holds true for the 350. You can validate that by looking at the administrative port speed options (ie, what are your choices other than "auto") to see what is possible. If they, too, offer more than just 10G, you should be able to insert the MGBLX1, and if it doesn't autonegotiate down to 1G, you can always manually configure it for 1Gbps.
