This is a great question that gets to the heart of what Delphix Data Virtualisation can offer, particularly in the Test Data Management space.
The Delphix Virtual Database (VDB) can be treated as a normal database. I like to say that it looks, smells, feels and tastes like a regular Oracle database. There is no difference; you can back it up with RMAN, your applications can connect to it and you can use all of your regular database tools. In fact you can even use it as a source database to another Delphix Engine!
So, yes you can insert as much data and as many new database structures as you like. There is no practical limit to the data you can insert, other than the storage limitations of the attached storage. In terms of the size that new data will take up - that will be based on the compression ratio you are getting on the initial ingestion. We usually expect for an Oracle database 2.5 to 3 times compression, so the new data is likely to take up a third of the space normally consumed by a physical database.
You can subset and generate data using any tool that you use today and the referential constraints will be the same those that you have in your physical database and any application running upon it. The beauty with using a VDB for this purpose is that you can bookmark your modified, generated and subset data. By that I mean you can "snapshot" a version of your data so that at any moment you can restore to the version of your choice and more importantly you can effectively "share" that version with your colleagues. It really does change the way in which you view the data management issues and accelerates environment preparation by orders of magnitude.