I do not know much about web tech and currently, I am planning my own project implementing web app + hybrid mobile app.
Based on my research, it seems many people use bootstrap, on the other hand for mobile, ionic seems one of the most popular ones.
web: bootstrap +
1. is it easy to combine bootstrap to ionic?
2. Is there widely used tool that is integrated for web and mobile application development?
Bootstrap is meant to be a CSS library for creating responsive web, be it webapp or mobile. You can easily create a webapp using AngularJS as you mention, and let its responsiveness be driven by Bootstrap.
Ionic 1 is meant to be a mobile app development framework, not mobile web. It has got a lot more than just CSS styles; it creates hybrid native apps for Android, Apple and Windows platforms. Though it does just run fine on mobile screens as a webapp.
Ionic 2 has support for webapps so you can easily write your mobile app that can then be deployed either as a native app or on a web server.
Though on both versions of Ionic, you may choose to use Bootstrap for styling your elements but be aware that this won't replace Ionic's own scss. And it might create some unforeseen issues as you've pointed out.