I loved that issue. I really liked the direction they were going with CannonBall, but then they just kind of let that whole subplot die out. It would have made a lot of sense for him to have the potienal to be extremely powerful considering Cable always seemed to think he would play a large role in the future. I wouldn't say Cannonball even took Galdiator "on" at all.
Did Galdiator under estimate him? Sure, but Galdiator never intended to actually fight him for real. It would be like a human letting a ant crawl on his arm and then being suprised when it bites him, it might suprise you for a second, but your in no real danger from it.