Skinned meshes: Currently decals will kinda break with those. I mean yes, one could also skin the decal vertices, but then I would also have to create custom skeletons for the decals consisting of all bones of the skinned meshes covered by the decal (or only the needed ones, but then with custom animation frames and shit), also it is not really clear if such a decal should then stay the same and just move with the geometry or has to be projected again. Projection would probably be much easier, but also look wrong in most cases. The alternative to skinning the decals would be use CPU skinning for the vertices with the decal attached and update the decal for those. Would probably be the best solution, but still has the tricky part with projection vs following the movement.
I won´t touch this for now and would use a specialized solution for such cases like just modifying the texture of a character or something.

The decal actually is not attached to any scene node (unless you make it a child of something), instead it is just a scene node itself, can be moved freely and will effect everything in its range. Now if it is attached to another scene node and moves with it it again will effect anything in its range. So if this is not wanted, you will have to make sure it is not bigger than the scene node it is attached to.