Hi, and welcome to the avast forums.
Is the Server Edition compatible with Microsoft Cluster Server under Microsoft Windows 2003 Server Enterprise?
Can I install the aVast Server Edition in a Clustered environment server?
Yes. See also here:
http://www.avast.com/eng/avast_4_server.html#2Do I need to reboot the server after install of Server Edition?
Unfortunately, yes.
Also, how is the license scheme for Active/Passive clustering? Do I need to buy one license per server in cluster or one license per Active server in cluster?
As avast Server edition is priced per-machine, I'm afraid you'd have to buy a license for each cluster node.

Cheers
Vlk