I have multiple different APIs with different schemas serialised in XML or JSON which I need to output as a standardised schema.
Main features needed:
Is this the sort of problem Slumber with TastyPie would be best for?
Or are there a different libraries you'd recommend?
Thanks for all suggestions,
Alec Taylor
Are you simply trying to create a unified interface for all of you existing APIs? If so, are they using Django models? TastyPie would only be useful for you if you have existing Django Models. You could still use it without having Django models, but then you would have to write lot more of the api code yourself. TastyPie is a full-featured, drop-in API system. It is meant to simply included into your existing code, your attach it to your Django model and it will serve the data associated to the models. It provides all of the RESTful options: GET, POST, DELETE and PUT. 
Django-Piston also makes use of django models, but it does not solely relies on it. It provides a very basic interface (in comparison to TastyPie) and you would need to write all of the RESTful request handles yourself.
Both TastyPie and Piston have authentication and authorization system.