Where did the 20,000 tonne figure come from?
.... it would seem to make the assumption that we would be able to store cargo on Carriers separately from ships.
It may be that no off-ship cargo storage is available on Carriers (just like in any other dock).
True, there is no hint on cargo storage, or any storage whatsoever beyond our ships (and even storing more than one ship is, for now, speculation).
But on the other hand, what could be a "trading specialization carrier"? If it just means that the carrier would have cargo racks on outfitting, it would be a bit pointless, as anyone trading will already have cargo racks installed on his ship.
From my point of view, carriers bare minimum will allow storing our ships (all - or a high limit) and store our modules (also all, or a high limit). Beyond this everything is in the realm of speculation, but if there are "support ships" I assume they'll be for something...
Worst case scenario is that support ships is just a visual representation of the "station menu buttons" we unlock when choosing the carrier specialization. But that would make the support ships so pointless (why having them at all just for fluff - same fluff could be applied just to the carrier itself) that it really has to be something more.