Ok, so last night I bought RT (see http://www.fantasyflightgames.com/edge_foros_discusion.asp?efid=123&efcid=3&efidt=466957&efpag=0 )and spent about 5 hours devouring the book.

The one thing that is not clear to me is cargo space. Do ships not have a cargo hold for "loot" unless they take one of the cargo space options for their ship? Is it up to GM fiat how much a ship can hold?

nope they do, but you need the cargo components for endevours such as trade routes or bulk cargo. But in short all ships have cargo space just limited to mostly supplies to stay out in the void and maybe minimal extra space. You get the cargo components to have extra dedicated cargo space. On your second question yes it is up to the GM what max cargo can be since the concept tends to be abstract in nature.

Ships can carry cargo and passengers without dedicated Cargo and Passenger Components. They also have small craft without any dedicated Components to that effect.

Having dedicated Cargo and Passenger Components provides a bonus to Endeavours related to using such Components, and (per BFK) also provide added small craft to help move the cargo and/or passengers (4 extra space of craft per Component, aeronautic craft taking up half space).

Did Battlefleet Koronus ever get around to explaining what kind of default light craft/cargo space a regular ship is equipped with?

1 point of craft per 5 Space on the Hull plus 4 points of craft per Cargo/Passenger Component (but there has been a statement from Ross that some of these Components do not provide extra craft). Most craft take up 1 point, but light primarily atmospheric craft (like an Aquila Lander) only take up 1/2 point.

HEH. Interesting.