You can set it in the config file. (server.propeties)
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You can't change the seed in the demo you can only in the full edition.
No, there is not any good servers for Minecraft 1.6.6. In fact, there are no more 1.6.6 servers at all. There are only updated servers. My recamendation is for you to update minecraft. All of the servers, or the most active, always update.
Pokemon on Minecraft is a mod, not a seed. Search the Minecraft Forum.
A premium account is a Minecraft account that has bought the game. A premium account allows you to change your skin and play on multiplayer servers.
You would have to ask the owner of the server about that, it varies between servers.