For The Island, you can go there around level 16. or 30-something after you've finished first playthough. The challenges will be shaped to those levels.
I did this. Playthrough one till completion. Then Ned's Island. Playthough 2, start at Ned's first then go through to the Vault. The level challenge will be scaled best like that.
You can play Moxxi's at any level and anytime you want that quick(ish) fix as the levels will be scaled to the Host. That's the important part. Invite a level 50 to the party and he'll have an itchy trigger finger trying not to kill everything for you.
I just want the Hat!