As far as I know, currently there is no way to request a specific version from Skin Builder.
And concerning your second question, the skins are guaranteed to work with the latest version of Webix, but they are not backwards-compatible. However, if there were no major changes in the skin compared to the version you’d like to use it with, there should be no major issues with using that said skin, but I’m not sure if that’s the case with the later skins, as they’ve had some of the changes on the major side. All in all, you’d be using the skins at your own risk if you decide to try and use them in tandem with an older version of Webix.