I actually put some primitive homemade floats on a Royal 40T with an OS.40FP. It flew well. For a plane that size you'd need floats between 32 and 36 inches.

Other question: yes -you can mount floats under the wings. It is done mostly on low wing models. Imagine a GeeBee racer with floats instead of those big wheel pants. On high-wing models like you have, The struts would be long and either heavy or fragile (or both) and probably goofy. On the 40T, mount the floats to the main gear wires (two wheel collars will secure each float) and make a T-shaped piece of music wire to install in the nose-gear mount for the front float mount. You'll probably need a wire between the floats at the rear mount...it can be pushrod stuff-does not need to be heavy. On the Pica, get an identical aluminum set of gear legs and mount them at the rear of the cabin floor. You'll probably have to add a ply hardpoint to accept the load, but that's pretty easy.

The float steps go just a little behind the plane's balance point. example: If the 40T has a wing with 10" chord, the step would go at 4", or 40% of the chord.