What kind of services do Relatics and Revit have? Can you use webservices or restservices? Can you export data? I think the best way is to look how you can extract the information out of these applications. With Mendix there are more manners of getting the information into the Mendix app. I do not know Relatics or Revit and do not know how to extract the information from the.
In Revit their is also an option to export data to xml (http://thebuildingcoder.typepad.com/blog/2010/06/export-data-to-xml.html), but it is a script i do not understand.
In Relatics, you can create your own custom webservices (SOAP) to retrieve and store data (calling underlying reports in Relatics). These webservices can be called from Mendix.
You can also call the standard Relatics API (soap or rest) to read and update individual records.
Let me know of you need any help. We are a partner to both Relatics and Mendix.