Just to clarify, in order to gain the +1 you must have exactly six characters to gain IN THE FIELD right? So if I have six and overdrive becomes a pilot (leaving in the game five fielded characters on your side) The bonus is lost, is that right??

The same happens during mind control, bystander creation or ID summons, if anything (no matter how temporary) changes the count of your fielded characters, you lose the bonus until you get back to six, is that right?

100% correct, to my knowledge, the number of characters you control is an ongoing variable constantly being referenced by this trait.

You're right, except for Overdrive. He's still part of your force, even while in his vehicle. You essentially get 2 characters on your force if you have Overdrive and his charged up bystander out. Being off the map still counts as part of your force.

AEnima is correct. This is also where I feel compelled to point out (as this is the third time someone has said it for this specific trait) there is no "on the field" in Heroclix. There is on the map and off the map, neither of which have anything to do with whether a character is on your force.