Does the pilot in command always sit on the left seat?
No, the PIC does not always sit in the left seat, and there's no regulations about it. Most airplanes are configured so that the primary pilot would sit on the left, however there are military trainers (the Slingsby Firefly comes to mind) which have this on the right so that the pilot has the throttle on the left and stick on the right as single seat military jets have. Helicopters have the pilot on the right, not the left. 

In any case the PIC is not necessarily the handling pilot, instructors usually sit on the right and are often the pilot in command.