You're welcome. As mentioned previously: "If the index.xml file doesn't have the xref="0" set, the XRef to library option is allowed."
There's no need to add the xref="1" to any library, which will allow references as without being xref="0" specified, it's behaving like having set xref="1".
All plant libraries we actually sell on our website are done by third-party companies - like XfrogPlants by Xfrog, HQ Plants by 3d mentor, Archmodels by Evermotion, Plants Kit by Laubwerk and Plant Models by MaxTree (our company develops Forest Pack and RailClone, plug-ins for 3ds max). Let me mention, that any model inside of the 3ds max scene can be used as a Custom Object under the Geometry tab. So, if you Merge (or link as XRef) any model into your scene and then create Forest object, you can pick it as a Custom Object. This process can be done easily in automatic way. For this purpose we developed our Catalogs (HQ / Xfrog / Evermotion) linking all models of those model packages through Library Browser inside of Forest Pack.
I can confirm, that The XFrog Catalog supports the XRef to library option even without the optional xref="1" (as mentioned above, there's no need to specify xref="1" to allow referencing).
Hope that clears it up, but in case of any further question or doubt, please don't hesitate ask.