I think Azalea Japonica is just a general name for garden azaleas.

There are some 800 - 1000 rhododendron/azalea species and thousands of varieties. Satsuki alone already many hundreds of different varieties. Of 'Kurume' azalea, there are hundreds too.

I wouldn't know how to see the difference - all I know is that Satsuki and Kurume are suitable because of their leaf size.

More info about species and varieties here:
www.sonomahort.com/azalea_info.htm#kurume

More about Satsuki and Kurume here:
www.bonsai4me.com/SpeciesGuide/Rhododendron.html

And then there's also a north-american species suitable for bonsai, more info here:
kuromatsubonsai.com/azalea-bonsai/