If you can set the incoming and outgoing port SKYPE uses, then you can QOS it.
That's the problem - Skype will either use port 80, 443 or a random port (you can change it) for communication.
I guess you make sure everyone used the same port for Skype, that would help, but this isn't something I have tried.
Last time I looked at Skype's network administrator guide, they didn't have any real suggestions for improving call quality except that it can help if you have a public IP address and access is open to your Skype port.
