Hi there can you uninstall harts of stone after saving your game or anything or will that break something?

Why would you want to? You've bought it, presumably, and it's official DLC, so it's a serious question.

You saved your actual game before loading/starting HoS?

Well - you only have to start up the save, ignore that you ever downloaded and installed HoS.
You loose nothing but a bit of HDD space, but gain the option to actually go back to HoS if you will change your mind later on.

So yes: "why would you want to....?" :)
But yes, HoS has an entry "programs and functions", so it can be deinstalled.

I just want to know if it would break anything if you started a quest or something.

If you have HoS content in your saved game and then uninstall it, then yes it will break your game.