I was wondering, which of the crew skills are complementary? Like mining and smelting in WoW, which gets the components for which?
Here's a very handy diagram for how the crew skills relate to each other.
This site contains a useful list of the most efficient Crew Skill combinations for each class based on the companion skill bonuses: http://r2-db.com/guide/124/skill-choices-based-on-companion-bonuses
The following is a list of all the crew skills and their related Mission and Gathering counterparts:
Here's a graph that delineates gathering, crafting, and missions.
—from SWTOR Crew Skills Diagram on Google Docs