The only other character I can use is maxed out and I find maxed out characters to be pretty boring.
Out of all the DLC's it is by far the most difficult of the DLC's I'm just going through it now with my 50 mechro and these are the only enemies that have given me a challenge, the various types of witch doctors are a great variety to the normal enemies and their ability to heal/level up the enemies around them is great.
But to answer your second question, if you enter before finishing tvhm I don't think they will level with you, but more or less stay the respected level for that DLC. Once you beat TVHM and unlock 2.5 though I'm not sure if they will then scale to your level, I imagine they would just not the quests you have started. (Which I don't think is that big of a deal as the new unique weapons drop from enemies and aren't quest rewards specifically)
GT: Social Sin Fanboys = The bane of any intelligent individual