I am interested in developing a server software for ClassiCube, but I haven't been able to find any specification of the packets that must be sent between the client and the server. I did find one for Minecraft Classic ( https://minecraft.wiki/w/Classic_server_protocol ), but no information about ClassiCube and its protocol extensions (custom blocks, block IDs over 255, etc.)

1) Is the ClassiCube protocol a perfect superset of the Minecraft Classic protocol, much like C++ is to C, such that a server written for Minecraft Classic could be joined from ClassiCube?
2) Is there any decently written documentation about the extended protocol?

You can find the Classic Protocol Extension documentation HERE
ClassiCube uses the original classic protocol and this extended protocol, and is backwards compatible.