I’m looking for the same and maybe I can add some information, regarding the current state of fstop-dropbox collaboration :

You have to view a cloud picture in detail mode (or dia show) in fstop, to trigger the indexation. I can search for tags of all my dropbox pictures, I’ve viewed as a single picture. You can see which pictures are indexed, as they’re marked with a green cloud icon instead of a white one. Unfortunately this is very cumbersome for thousands of pictures.
@Support: Is there an option to auto-index all files from dropbox?

Keeping an eye on the amount of user data fstops stores on the phone, it looks to me, as if fstop can only index files, which are stored in full size on the phone. At least after having viewed and indexed 20 pictures as described above, fstop now needs more than 100 MB more on my phone. This would render the cloud index feature useless, as I would have to store all my cloud files on the phone.
@Support: Is there any option, to just store a thumbnail and the index data on the phone?