You should first consult the license of the icon set. You can also contact the author of the icon set for permission. Usually icon sets can be used in both commercial and personal project. However, you may not redistribute these icons in any shape or form.
No, you can download the icons absolutely for free without any registration.
None of the icons can be downloaded for redistribution purposes. You are welcome to link to the the page on our website displaying the icons, in order to direct others to download them.