I have all of the DLC but have never done any of them and I am wondering when is the earliest I can start each DLC without missing out on anything important or really good, and not get constantly slaughtered on normal difficulty? Also list any specific skill levels recommended for challenges please.

You can start as soon as you wish, before going through each DLC - the prompts will tell you recommended level requirements and give you the option to either go through with it or come back at a later point.

Also it's a good idea to do each DLC in the order of the release - so as not to confuzzle yourself in the back story.