I want to start a new project and im not sure, if im using the best setup/approach. The project is kind of a list directory without high computing power needed.
I planned to build an website with Rails Api and AngularJs (+Bootstrap) for normal devices and mobile and use the same api for apps too.
Therefore i planned to use the IonicFramework (+PhoneGap). Is this a common approach or are there any best practices i should consider?
This is the standard way to develop web/desktop apps and mobile apps. You are approaching this correctly.
Use core AngularJS + Bootstrap for web/desktop app and Ionic
Framework for mobile
Sharing the backend Rails API makes sense. Typically the Rails API would be designed with REST principles in mind which makes it easier to consume (probably using Restangular) via web/desktop app and mobile app
If you follow standard AngularJS conventions to create a separation of concerns (controllers, services, and views) then you will be able to share quite a bit of JavaScript code between the web/desktop app and mobile app and easily override functionality to customize for devices, if necessary.