One-man-army is not a trait for 1 guy party, it is for a one-player-party. If the game would be made 'reasonable' to play it with only one guy, the fights would be even more boring with four guys after level 4.
There is a lock that prevents accidently pulling your skills out of the bar. Open is and you can exchange them at any time.
The biggest problem with fire and poison is less the damage itself and more the insane chain reaction that can happen because of their interaction. Something that will never happen with Water and Air. It's also the main reason why environmental effects can be deadly for the own team.
