I am working on an article for Reallusion Magazine that concerns some of these questions. In my testing, it does appear that custom or third-party kits need to match but I'm not through with testing yet. I was able to successfully convert a Stonemason (Daz) hallway kit to use as a hallway or building exterior and the only thing I had to change was the height of some items and that can be done in the packager. I am still testing out different 3rd party packs for more info on this. Hopefully, I can cover some of your questions in the article.
FYI for other users: Below is a custom package I made from props in the French Style Pack. I used these props to learn how to use the plugin packager as I knew the props would be sized right and it helped me to understand the plugin.
