I don't have any experience of six player games or setting up multi-player generic (e.g. LoME) scenarios, but here are some thoughts on the matter anyway.
I have played some of the Fellowship in Moria scenarios with my beginner friends so that one player plays the Fellowship and three players each have a group of goblins with a captain or cave troll. That has worked pretty well, although there have been some "Oi! That's MY goblin!" moments.
Then there is also the Amon Hen scenario group in which you can have Aragorn, Legolas and Gimli coming to resque of Boromir, but those scenarios work best when played in different rooms.
We have also played some Rohan vs. Isengard scenarios (e.g. Scouring of Westfold from the old Two Towers book) as multiplayer games. One player could for example play the Rohan cavalry and another infantry. I think an important thing has been that each player has a coherent group of warriors and at least one hero, and the group has some clear objective that is important to their whole side.
-- Pasi