RE: Tuttle/Deep Tuttle Cock Up
Roger will give you more detailed answers but I can already tell you that there is no problem to use Tuttle fins in deep tuttle boxes. You only have to find longer screws. The opposite is impossible of course.
As for the fin length, 28 cm seems to me very short for this big board, but for a very small sail and high wind it could, perhaps? work . It will be difficult to head upwind in gusty winds.