Q: Can you fail-over if the Central node hosts the Repository Database and the apps?
A: Yes, but there's little added value to this style of deployment.
Q: Do you have to add a new node to allow the node to be a fail-over candidate?
A: Out of the box, yes. If you're comfortable with APIs then you can elevate an existing RIM node. See article Elevating an existing RIM node to fail-over candidate in June 2017 or higher in our knowledge base.