I have put floats on most of my planes at one time or another, up to and including my 80" Midwest Extra 300XS in one of the photos attached. All fly well with well designed floats. Also the Balsa USA Northstar is a popular seaplane, but if it is your first build I would not suggest that this be the one for you.

As to power requirements, keep in mind that the floats are creating as much, if not more, drag than an extra fuselage, and power accordingly. As an example, the floats I have used of the Midwest Extra are the same ones that I have used on my Midwest 80" Citabria, and each float is about the size of a .60 sized plane's fuselage. Therefore, with the Citabria on the wheels the Saito .91 is a good powerplant, but when the floats are on the plane it wallows unless running at full throttle.

For a .40 size plane that will have floats on it I would strongly suggest a .60 size engine for the reason above.