I agree, that is a great looking plane.

Vertical stabs on the floats are not used because their moment arm is so short that they are only 40-50% as efficient as the same area (and presumably weight) added at the rear of the fuselage. You have plenty of side area in the tail without them so they probably aren't needed, but on the other hand, they probably won't hurt anything and if you like them....

The size you have given is quite small to carry the added weight of the floats. I'd watch the wing loading.

Jim