Some locos do not run well with others. I've been out of the hobby too long to give any decent examples. But recently at one of the store sites I saw a review of the new engine and they talked about how the engine ran with other locos.
That depends on the loco. If its actually wired, you may be able to do it with a soldering iron without dismantling the mechanism. If it's a split frame design you'll have to take the entire mechanism apart and turn the motor over. If the motor contact tabs are different lengths you'll have to swap them, which means pulling the brush caps off. If you pull the brush caps off don't lose the springs and brushes and be sure to get them back in right.That sounds like a great idea. How tuff is it to do with n scale loco's?