if you build on the xowiki-includelet infrastructure, i would recommend to use the same mechanism that the other 50+ includelet classes in xowiki use, namely require_YUI_CSS and require_YUI_JS. These functions allow you to use either locally installed versions (per default from the ajaxhelper package) for internet-free execution, or to use versions from the Yahoo content distribution network (yui.yahooapis.com). This is controlled via the ajaxhelper switch (it does not use the package - such as the ajaxhelper loader etc - just the files).
The require_YUI* methods provide an xowiki-specific configuration for all YUI needs in xowiki and friends. These functions control as well the YUI version numbers. The preconfigured version is YUI 2.7.0. In general, i would not recommend to mix YUI versions. Not all newer YUI versions are fully upward compatible, i had to step back e.g. from 2.8.* to 2.7.0 due to changes in drag and drop. If you use multiple YUI components from different YUI versions on a single page, you are probably asking for troubles.
