I fail to see where sixguns would solve the problem of multiple hopper charges. Most, if not all, Hopper attacks come in from cover or the side of the board and end in cover outside 10". No reactions are allowed at that point.
Exo's jump 15, one of the best in the game. Keep mobile and try not to let them corral you into a corner.
Face it, when Hoppers attack, MI die. There is little to nothing that can be done about it. Accept that MI will die, try to minimize the loss with supporting squads of Exo's and move on.
As for SixGuns vs. a single Firestorm. IMO the Firestorm wins. 4d10 direct fire vs. 3d6+1 is a no brainer. Most turns end with MI players on ready, so they only get one action anyways. If you are concerned with only firing it once a turn, buy several Ammo Dumps and hopscotch back and forth to them. Spend a round on one and you can fire twice.