Well, number one, you're going to need mods. Hera and Chopper have a bunch missing. And of the characters that do have six mods, a lot of them are only at level 1. So that is your biggest problem: You will need to make sure they have full sets of levelled mods. Take some time to learn about which sets and primaries are good for each characters.
Also make sure their abilities are as levelled up as you can manage; Hera's lead makes them share uniques so having those as high as you can go really helps. You may need to be a higher level for this, but if I recall, mid-to-high 70s should be enough for t7, let alone t6.
So for tier 6 I would think mods is your major concern.
