No, don't change the downthrust because of the floats.

Engine downthrust is generally tuned with regard to flight characteristics, not taxiing considerations. The nosing-over experienced with floats, in my experience, either comes from overloaded floats or poor float design. There is almost no drag made by the floats at idle, but as the airplane speeds up, the floats push a bow wave in front of them. This is the maximum drag point. If the float fails to rise over the bow wave, it makes a lot of spray as water hits the prop and the plane could refuse to go fast enough to get on step. If you have ever water skiied, you have felt this as the boat pulls like bloody hell until you pop up on top of the water (on-step). Once you're up, you can one-hand the rope and sometimes it will even go slack. Our airplanes accelerate noticebly once they pop up on step, due to the decresed drag.

I suppose if I put on my Engineer hat and did the equations, having floats instead of wheels hanging below the airplane <might> let me use slightly less downthrust, but in my experience my planes don't seem to fly much differently with floats than without them. Floats have a lot more frontal area than wheels, but have a much lower coefficient of drag, so a big float might not cause any more drag than the wheel it replaced.

Downthrust is used to compensate for (1) having the thrustline below the center of drag, (which is usually the wing) (2) to compensate for trim change as the wing makes more lift at higher power/higher speed settings. My telemaster balloons at high throttle setting whether it has floats or not, and telemasters have a lifting stab to prevent that.

Flying boats such as the Seamaster have the thustline above the wing and not below, so they need UP thust to keep the engine from pushing the nose over as you add power. This was one of the problems with the Herr Aqua-star: the engine pod was on top of a long arm and it got a lot of leverage to push the nose down. Still, that's for flight and not for taxiing. The submarining thing on the Aqua-Star was fixed by adding chine-rails to the sides of the hull to get more lift from the water.