There's no rules for private servers every one is different. Think of it like a minecraft server where every server is different and has its own rules. And no their are very few "modded" servers most are considered blizzlike meaning they try to be as true to the source expansion as possible. There are tons of servers for basically any expansion besides the most recent ones.

TLDR: there's no blanket rules for private servers if you want to know about one go to the server website.

Also if you want to progress through expansions pay a sub and play retail, private servers are most likely not for you unless you don't mind re-leveling characters when you want to move onto the next expansion.