As far as I know, all new saber stances work with JA+, so I don't think you have to worry about that. :)
Well, like Movie Stances. They aren't and I'd like to make them so...
What do you mean? From what I know, a stances pack changes a few animations. How does it not work?
Change the name of the stance pack's PK3 file to begin with any letter after J, then put it in the same folder as the JA+ GLA anims pk3, be it japlus or base.
That will load the new animations in preference to the JA+ animations.
Just expect some side effects.
I don't know how, and I don't know why, but this is totally Sheep's fault.