There are 'light chains' which are mentioned in various contexts:

   * immunoglobulins 
   * amyloid 
   * myosin

and more.  Are these light chains all the same thing?  That is, a
subunit which is usable in many contexts; or is each one different
from the other?

Can you clarify:

Are you asking if the light chain of immunoglobulin is the same light
chain found in myosin etc?

If so, then the answer is no. 

It is a generic term for the lower molecular weight subunit of a multimeric protein.