You can choose 1 battleship variation for every slot in your formation - ships of the same class but a different type cannot be mixed in one slot but can take different slots.

@oliverschmid Ships of the same class but a different type cannot take different slots. e.g. you can't make a Corvette out of the Frigate. I think it was possible in the beginning, but now you can only select variations of the displayed ship class. You can't select an other ship class any longer.