Fortunately, Notion allows you to use whatever sample library you like, and there are even presets for a couple of them. Presumably, the presets allow those third-party libraries to work seamlessly with Notion, just as if you were using the Notion sound set. But reading through a variety of posts, both here and in the Custom Rules area, it seems clear that this presumption is at least partly false.
Not being one for shelling out a few hundred dollars on a new sample library and then finding out that it isn't as fully integrated as the Notion library, I'd like to lobby for better integration of the already-supported libraries, and to make a plea for the (proper and full) integration of other libraries.
I understand that it can quickly become a tall (and largely unprofitable) order to write code for the support of competitors' products, and I realize that some Notion user is always going to be displeased that his/her library of choice is not supported directly. Still, the forum traffic dealing with questions and problems concerning the use of Miroslav, EWQL, GPO and others indicates that there is a real need here. (Oddly enough, I don't remember seeing much chatter about Vienna Symphonic, which is featured in all of the video tutorials on custom rules. Could this be because the product is so expensive that it is out of reach to most Notion users?)
I am not a gearhead. I generally look for technology that suits my needs "out-of-the-box," and that's why I am so happy with Notion. But when I see that pcartwright is writing his own rule set for GPO and is still not satisfied that it is complete, maybe he needs some help. All I can do is add my voice to his complaints and hope that this post is not sent in vain. High on the list seems to be better support for keyswitches and the addition of more instrument IDs. Also on the list, but perhaps not as likely to be in the offing this season, is support for other Garritan libraries, such as Jazz and Big Band (with its superior saxes and brass), and the recent World Instruments. Something as simple (to me!) as being able to switch a staff from, say, a Notion trumpet to a Garritan trumpet to see which works better for a particular file, would be really wonderful. As it stands now (and as I understand it) making this switch does not guarantee that the notated part plays back as well as it should. Dynamics, legato, and basic articulations are handled, but that's about it. And please note that I would prefer to make this a Notion-only thread — not a thread about the wonders of Reaper and other master-slave relationships

The ability to mix (approved) sound libraries without having to figure out how to add mutes and such or jump through programming hoops would not only make Notion a better application, it would actually improve the usability of the standard Notion sound set, precisely because it would exist in a more varied environment where its qualities could shine brighter than they do in a Notion-only sound world.
Best wishes,
Thorrild