It's pretty much got to be all natural 4 star toons to make Tier VI. Might have to level some up to make the cut. I try and max out 4 and 5 star for maximum essence.
One and two star are just pit fodder, but works best when you use at least 9 two star and a one star to get three drops. I think all the tiers work that way. If you make the minimum, you get one drop. Pass a certain threshold to get two drops, and another threshold in that tier gets you three drops. So, to me it's better to save higher level toons to go for the three drop threshold.