There should a formalized schema (maybe a .xsd) for probe definitions. Validations on XMLs should be enforced where a probe definition is supplied (as a file path or via JMX).
Additionally, the way probe_template.xml is currently structured is a bit sloppy and should be fixed. Eg. multiple <field> not encapsulated in a <fields> parent, and multiple <parameter> not encapsulated in a <parameters> parent.