If you do end up using the mouse only for character selection, then it will only frustrate players to have to switch back and forth.
You can always use a key, like X or P or something to rotate through characters. Interaction wouldn't be too bad. And, who says you need to keep both hands on the keyboard? You can always have one on the keyboard and one on the mouse.
I'll explain a bit more
I was thinking about maybe try a cool action game after finishing my current one.
This game could have like 5 or 6 characters on the screen doing stuff
The cool part is that each character have specific skills like jump high , speed , attacks etc
I was thinking about how mix a puzzle game with some fast action paced.
So i decided to think about using mouse to select guys like you select units in a wargame
The problem is that , it's not a wargame and the view is like mario .
The game could be like a "lost viking" clone ...but instead using a button to select guys , you use a mouse , i think it's kinda more intuitive and innovative ?
I dont know.