The delay in the assist is cause its the WoW servers who have to update what your focus is when you pick a new target. The assist really isnt the problem... as long as I am reading that right.

Even though you have auto-complete or whatever on in Jamba you still have to make sure the quest giver is active for the slaves. Then it will replicate your actions on your master to your slaves. Note that what Keyclone says below about the looting and vendors will also work for quest givers.

As for classes, general concept is that the more variance you have in your group's classes the more complicated things can become. If you are looking for something easy to multibox the general suggestion is to make 5 Shamans and go from there. If you are looking to do all melee characters there are a few posts in the forums ya can search for that contain some helpful pointers for getting everyone into melee range.