Datatypes

I'm writing an ontology with properties pointing to literals
representing the following:
- media types (eg: "application/xhtml+xml")
- media ranges (eg: "application/*")
- fully qualified java-class names (eg: "org.example.tools.MyTool")

While I could use xsd:string I think it would be better to use custom
datatypes to have literals like
"org.example.tools.MyTool"^^http://example.org/datatypes#javaClass.

My question:
- Anyone knows a place where such datatypes are already defined?
- Datatypes are typically defined in an XML-schema, is there an ontology
to do so in RDF?
- Is it a good idea to define the datatypes in the same namespace as the
ontology using them?

Cheers,
Reto