I want to start the the hearthfire DLC since I plan on adopting and building my own house (and get the steam achievements too), but I'm scared that my game or my saves might get corrupted by installing DLCs in the middle of the game. I already defeated Alduin, completed thieves guild, completed dark brotherhood, and now doing college of winterhold. Will installing dlcs (Dawnguard and Dragonborn, Hearthfire) mess up my game?

Nope, you'll be fine. It might mess up your mods if you're using any, but it doesn't sound like you do.

I wouldn't think it would mess up your game.

Welcome to the Forums. http://images.uesp.net/c/c4/Fishystick.jpg is for you.

Nope. You should be fine.

No, not really.

Though I've heard that your saves are less likely to get screwed if you don't overwrite other savefile, making a new one instead. So if you're worried about corrupted save files, you could do that. Although I'm not sure if it works.