I guess you could do that. There may be a function or two that exists in both plugins. It's not really a scenario I support.
And yeah, there's no way to adjust the volume of the layers without my plugin, because base RPG Maker wasn't designed around having layers. (Except the Fade Out command, which I did overwrite to fade out all layers at once.) And... I actually realize there's a scenario involving adjusting layer volume I never tested. I'll... just check that real quick.
EDIT: Tested it. Found a small issue. Updated Soundtrack Manager to fix it. Thanks for somewhat indirectly making me realize it might be an issue. *ahem*
If you try to play the current base track at a different volume, either through Soundtrack Manager or the built-in PlayBGM command, it will change the volume only of the base track. If you use PlayBGMLayer with a layer that's already playing at a different volume, then it will adjust the volume only of that one layer.