Since Rails 3.1, jQuery is the default javascript library. Prior to that it was Prototype (although jQuery was still an easily configurable option). The addition of any other js libraries or js development frameworks is pretty easy.
I wouldn’t call using multiple js libraries as counterproductive but would need a really good reason for doing so since jQuery is pretty robust.
I’m just going to assume when you use the term 'jScript' you just mean "JavaScript". If you meant some sort of weird Microsoft thing, then I don’t know.
As was mentioned, RoR ships out of the box with support for jQuery *library* (not a framework!). Mixing libraries is not usually a good thing to do.
There are also frameworks, but Rails doesn’t have an opinion on these, which it really shouldn’t:
- Backbone.js
- AngularJS
- EmberJS
Coffeescript is an advanced dialect of JavaScript that is much more object-oriented, and (I think) easier to write.
What plays best is often a matter of client-side application architecture, how you want to approach things, and how much of an opinionated framework you like to work with. Angular is certainly a viable option. Writing your Angular code in Coffeescript and placing it in the app/assets/javascript/ folder will make Rails very happy.