Hi, Forest lets you to import the catalog items as XRefs, insted merging them in the scene. This is a good alternative to keep low the scene's size, without using proxies.
Unfortunately this is not possible with XFrog, because their plants are stored as a group of objects, instead an unique mesh (which is the format expected by Forest). So, all library items which are not a single mesh, are attached together once they are imported into the scene.
I've done a quick test using a proxy as library item, and it's imported corrrectly. Would it be possible that you used a group of proxies for your library, instead an unique object, and that is causing the problem ?