If you read the rules, it states you may not hinder a robot's progress on the bridge or the balance.
Meaning, so I would read it, that if you can take the ball w/o unbalanced bridge/goals, it's fair game.
However-- one slipup and the goal goes flying unbalanced, that's penalty time for your alliance. So you have to ensure it's worth risking the ultimate loss to take their ball.
But, if you've been practicing, it shouldn't be a problem
BCII will be very fun. T190 has been reading the rules to be sure we're completely up-to-snuff on them. Playing by them will make for so fun matches.
As for spare goals-- I know we get what FIRST gives us... our own team goal is ducktaped together cause we broke it when we knocked it over once.. so we'll see ;-)