ES6 classes and stateless components (aka arrow-components) are now standart things. You can continue use React.createClass, but may be this will be deprecated (however, I'm not sure about it)

For learning RN... they move fast, better is to create pet project from fresh RN generator (CLI: react-native) and go on.

no, it's not. I actually enjoyed working with React ES5 "classes" more than ES6 (but I had to switch cause of work standards)