I know the rule book indicates picking one of two randomly chosen characters, but just to be clear...

1. Are the character sets intended to be specific to each expansion?
2. If we're using all 61(?) characters, do we still only choose two?

The scenario I'm specifically thinking of is if we're playing any mode that has characters that are tailored to that specific mode, the probably of getting a character that is useful for that mode drops substantially. Is there any ruling on this?