What's the relationship between Documents, XSDs and Collections?
Yes, you are allowed to create multiple schema namespaces within a collection (in the Examples section of CREATE XML SCHEMA COLLECTION, see B. Specifying multiple schema namespaces in a schema collection).

But you can only associate a single XML Schema Collection with an XML field or variable. So whatever schema definitions you might put into a collection should all be relevant to the XML documents that you are going to store in the XML field or variable that will be assigned that XML SCHEMA COLLECTION.

If you will have different XML fields for different types of entities, then I would think those should have their own XML SCHEMA COLLECTIONs.

For more information, please see:


XML Schema Collections
Requirements and Limitations for XML Schema Collections on the Server