RE: GP CUB.20 ADDING FLOATS
I have one, it is in my avatar and there is another thread about it in here. It handles great on the water. I don't know about your plane but mine has always been a little tricky to fly in the air. By that I mean it will snap on you real quick if you aren't careful. The floats made it slightly worse but it's definitely flyable, although not a trainer. I think mine is like that because it's slightly tail heavy and I have the control throws set to the max so it's sensitive. Also the floats come with a ventral fin you are supposed to put on the bottom of the fuselage to make it more stable, I haven't put mine on because I think it's ugly but it would probably fly better.
I like the plane alot better on floats! It is a blast to fly or even just taxi around in the water. I built the GP .20 floats and they are pretty straightforward. I would recommend lubing the water rudder cable with some silicon so the water doesn't rust it. Also I have a OS .25 on it, I wouldn't recommend anything smaller. I think a Satio .30 or .40 would be a perfect match.
Good Luck