In general Pidgin plugins are not compatible; libpurple protocols,
however, are generally compatible but must be recompiled. How difficult
this is depends on the operating system and whether the protocol plugin
requires any additional dependencies or not.
I haven't tried to compile the purple-hangouts protocol, but I suspect
it requires some additional requirements (glib-json, maybe?) which might
be hard to compile on, e.g. Windows.
Generally the process is to compile Instantbird, then add the additional
protocol into the purplexpcom (libpurple) source tree and recompile. You
can then pull out the results and create an extension or just use your
own compiled version!
--Patrick