Just click and drag their portraits to the position you want. If that does not work for you, you have some bug. I'm playing a Tutu game right now myself.

edit: Unless you meant keeping them as the leaders of the party but changing the formation so that the fighters go first. Right click on any formation to get more formations. There aren't really many good options for keeping characters in the first slots "back", however.

I just use a scout or always send in my toughest tank first. Takes a bit more micromanaging, but w/e.