Hi, i'm afraid it's not possible to use nested Forest objects in the library. You must collapse the inner object to mesh.
Regarding the raycast error, probably your objects are placed too far of the scene's origin. Since Forest loads their own objects at (0,0,0), that is causing a huge bounding fox.
If this is the problem, there are two ways to fix it:
a) Move all your objects near of the origin.
b) Unlock the hidden layer "forest_templates", move all items near of your objects, and lock the layer again.
That should fix the warning message.