I have ~1.5TB of photos, stored in Dropbox.
Yes, by default, when you create a new library, you can specify a folder that is going to be continuously scanned for changes (additional folders can be added later). Anything within such a scanned folder will stay in place and will not be touched by the automatic organization functionality that keeps the folder hierarchy within the library folder tidy. You can also still select existing folders to be treated as events without moving them into the library, and moving them around or renaming them in the file system will be detected automatically and the library catalog gets updated accordingly.
The main caveat is that once you use the built-in synchronization to share the library with another device, the photos will be placed into the library folder on the second device and event folders will be stored separately from the other files. If this turns out to be a use case of interest, we will improve this. It’s mainly a matter of prioritization for us right now which organization schemes receive “first class” support, but there are already a number of improvements planned.
As long as only the photos are in the Dropbox folder, there shouldn’t be any problem at all. If the Aspect library folder is also stored in Dropbox, we are using a “lock file” as a safety measure to ensure that only one device is modifying the library at a time, so in general this is also a pretty safe setup.
The only real exception is if one of two computers is not connected to the internet and both access the same library in Dropbox, the lock file would then never be seen by the other computer and if both modify the library, eventually changes made by one of the two would get overwritten by Dropbox’ synchronization process. This is more of a fundamental issue with Dropbox, but unless forced, it shouldn’t be a problem in practice.
That should be completely fine. There will be a thumbnail/preview cache in the library that typically takes up an additional 2 to 3 percent of storage space. We regularly test with a library of 1 million images to make sure everything is usable at that scale.