A new recruit has some questions

When you'll end Vanilla main quests and reach LvL58, Borderlands, Moxxi and Robolution will automatically Scale. For Knoxx and Zombie you'll have to finish their main quests for them to Scale.

All DLCs do work like Vanilla, meaning 2 Playthroughs and except Knoxx All of them have goodies in Playthrough 1, so don't expect to skip it if you intend to get all your Skill Points and SDU.

As For Farming well... That's another story...

To Start with if you get Knoxx you'll be able to reach LvL69. Problem is LvL69 weapons can only And only be found in Vending Machine as the Purchase of the Day, 2 Midgets in Knoxx and Loot Goons in Zombie Island. That's it... So you'll never find LvL69 Machine Gun nor Cyans (New Weaps and Shields in Knoxx).

You can Farm for Boss Drop LvL68 in Vanilla and Knoxx, Vending Machines in All Pendora (exception for LvL69 Grenade Mod that don't Scale in Knoxx nor Robolution), and Knoxx for Oranges (they are easier to find there) and Cyans.

In Vanilla you can go to Crimson's Fastness for Chest farming, when done quite and come back cause changing area won't respawn the chests. But if you want to get serious Farming then you need Knoxx nothing beats it in therms of farming.

Another Option on Bosses that don't respawn or unique quests reward is to quite without saving. When the reward window appear and the reward isn't that good just do that. You can as well use a copy of your toon on a USB key so that if something get wrong you don't loose everything, but beware those devices are fragile so be careful... Just use a Mule, create a non XboxLive Character, then in splitscreen transfer it to your toon.