The mind of the face down characters must be 20 or less. At this point Thrall of the Voice might also be face down but it's effect would not yet be in play until it is revealed along with the starting characters. After the starting cards are revealed, the 6 mind character will have 5 mind.3) Place one to five starting characters (no Wizards) face down in front of you. The combined mind attributes (see page 19) of these characters must be 20 or less. You and your opponent reveal your characters simultaneously, placing any duplicated characters into your play decks.
So, any 6 mind character would still count as 6 mind when deciding your starting company.
The CRF confirms this: "Your starting company can only contain 20 points of unmodified mind."