The M is for mega so yes you can plug your mega ducer in that plug. The DB is for the high speed 2d ducer. You can mount the si ducer from the helix 7 externally to the terrova. You can run an Ethernet cable from the 7 and the terrova back to hub. Run an Ethernet cable from the 9 back to the hub. The appropriate dongles will be needed on the ff ends of the ethernet cables.
Now if you plan on having the 7 plug into the built in US2 ducer of the terrova then you will need the dongle for that as well.