1. http://jquery.com/download/
2. Yes, most of them should.
3. Test for the functionalities that you use, rather than testing for all.
4. Just load one version and not multiple, and use minified version.

Also check How well does jQuery support backward compatibility?

1. http://jquery.com/download/
2. Yes, most of them should.
3. Test for the functionalities that you use, rather than testing for all.
4. Just load one version and not multiple, and use minified version.

Also check How well does jQuery support backward compatibility?