I created a lengthy 6.1 follower theorycrafting post yesterday and in doing a bit more research, I learned you can 100% every GR mission with max Scavengers/GR return using only 4 followers.

A few things to note:

• Deadly Minions never shows up in any GR mission currently in-game.
• All single follower GR missions are single ability counter only. Since there's 9 total ability counters but we remove Deadly Minions (previous point), 4 followers can cover every ability for single follower GR missions. The exact ability combination per follower matters when you want to optimize 2 or 3 follower GR missions.

Here's an image of all multi-follower GR missions, their ability counters, and how to cover them all in 4 followers:

http://imgur.com/6sfpkKI

Basically, you need the following ability combinations on a Scavenger follower to setup this 4 person team:

• Massive Strike Group Damage
• Danger Zones Timed Battle
• Minion Swarms Wild Aggression
• Magic Debuff Powerful Spell

The thing to realize is that not all specs/classes can roll these combinations. This is where it gets a little tricky. Below is a list of the classes that can roll these combinations:

• Massive Strike Group Damage: BrM Monk
• Danger Zones Timed Battle: Feral/Balance Druid, Hunter, Arc/Fire Mage, WW Monk, Priest, Ass/Com Rogue, Enh Shaman, Arms/Fury Warr
• Minion Swarms Wild Aggression: DK, BM Hunter, Arms/Prot Warr
• Magic Debuff Powerful Spell: Frost DK, MW Monk, Prot/Holy Paladin, Resto Shaman, Aff Lock

EDIT: /u/scavthrowaway has generated all combos that'll cover the Scavenger missions which includes teams that don't need a BrM Monk: http://pastebin.com/5G3E68pL

Hope this helps!