The Skype protocol in encrypted. Encryption has not been broken. Therefore, Trillian can not directly support Skype. The SkyLlian plugin uses the Skype API to add limited Skype support to Trillian. However, the plugin is in an early development stage and due to its concept Skype still has to run in the background. Ceruleanstudios stated that they will not support Skype until it is possible to directly integrate it with Trillian (e.g. without running Skype in the background) as this would defeat the purpose of Trillian.

Yes. You can download and use Trillian Basic for free. It does not contain any ads or spyware. If you would like to have additional features, you can purchase Trillian Pro.

No, Trillian is currently only available as a Windows software. Trillian Astra will introduce a web-based version that can be used in a browser (and therefore also on Linux or Macintosh computers). The core code of Trillian Astra compiles under Linux and Mac OS X, but that is  just a first step  and  does not mean that a usable  Trillian version for these systems  can be easily created. 

An alternative for Mac OS X is Adium X.

No, additional plugins can only be used in Trillian Pro.

While only Trillian Astra is specifically build for Windows Vista, Trillian 3.1 runs fine as well. However, you might experience a few issues.

Windows Vista might switch from the Aero Interface to the Basic Interface when you start Trillian. This is not caused by Trillian but by Quicktime (which is accessed by Trillian for some features). You can prevent this from happening by changing the Quicktime preferences. Go to the Advanced tab of the Quicktime preferences and set the Video mode to Safe Mode (GDI only).