Do captains and first officers always fly together or are they mixed? I was wondering if the same captain and first officers always fly together for some time as some sort of a team or are they mixed around in every flight? I suppose if you have one plane that is the case but what happens in huge airlines such as emirates? Wouldn't it also be a drawback always flying with someone that you do not actually know?
In airlines where pilots bid on monthly lines it is conceivable that a pair of pilots fly all month together. However, trip trading and reassignments rarely let this happen. It is far more common that a crew only stays together for a single "pairing" that lasts between 1 and 4 days (domestic 121 USA). 

It's not a drawback to fly with new people because training emphasizes standard flying according to the FOM. Ideally, every pilot is exactly the same.
  Practically it is close enough that you can drop any two pilots from the same airline into an airplane and they more or less operate it exactly the same way.