So, first off, does this work with custom tiles? And secondly, will I have to make a new project with the same events and maps (etc.) to get it loading?
Viewing post in MZ3D - 3D Rendering Plugin for RPG Maker MZ comments
Yes, you can use custom tilesets. Just remember that you need to specify how each tile should work. You can define them in the "Note" field in "Database" -> Tilesets" and in the bottom right corner. You have to do this for each tileset. As for having to create a new project, no, it shouldn't be necessary. However, if you have created your project in 2D, I do recommend creating new maps with the old ones as a blueprints considering that it doesn't translate well from 2D to 3D.