DC Motor and Relay in series with each other
Could anyone kindly please point me in the direction of a good
  learning resource...?
  


No. That is not what we do here.


  
  When a motor says '6V - 0.5A' is this normally the peak current? i.e.
  does it guarantee that the inrush current on start up won't exceed the
  0.5A value?
  


Most probably not, but many motors described on the internet are very poorly described. The one linked to the question is a good example of what not to buy because of the poor description.


  
  Have I got this wrong?
  


Yes. Essentially, a permanent magnet DC motor with a commutator behaves like a resistor and inductor in series with an opposing voltage source that is proportional to motor speed.


Whatever?


The motor current should go through the motor and power supply. The relay should not be in series.


would it be sensible to use a higher voltage supply and add a current limiting resistor?


No. It would be sensible to buy a motor and supply that are more closely matched and use an electronic circuit to control the motor speed by controlling the voltage. Putting anything else in series would increase the speed variation due to load variation.