Adam,
True – quotas can be applied at the qtree level (the q in qtree is for ‘quota tree’ from its initial purpose years ago).
So if it’s the only one within the data set that you want to have quota applied, yes it will have to move into a new qtree, possibly by itself.
If you want it linked or joined with the rest of the directory structure you already have (ie: if it would be nice to have it still placed in the same directory structure you already have), then you can take advantage of symlinks. Search “symlink” in the communities for a picture of what they are, can do, and how to implement.