InfoPath won't let you dynamically add/remove controls, but they can be
hidden or shown using conditional formatting.

If I am to understand your problem correctly, you intend to deserialize the
form XML into the appropriate .NET class instances. Unless you are using
special attributes on your class definitions, I presume that your schema
would need to use different element names to represent different derived
class types so that the deserializer can recognize the correct type of the
instance. You expect InfoPath to be able to show the correct controls
depending on the type of the instance; i.e. if you extend a type by adding a
couple of properties, the UI for the derived type would include a couple of
additional controls bound to these properties.

You might be able to accomplish this in InfoPath using the following
approach and restrictions:
- Use a single element name for the root node of the document
- Where a parent node can contain instances of different complex types,
define an xs:sequence that includes an xs:element for each possible type,
making sure the minOccurs attribute is set to 0.
- In the InfoPath form, add an optional section for each element that
contains the appropriate controls for an element of that type.

To deserialize the XML, you will probably need to include some custom
attributes so that the different element names are deserialized into the
correct types and mapped into the appropriate property of the parent instance.