Wetbird, for what you are doing the lightest waterproof covering will be shrink film, of whatever brand. I only use Koverall on models over 80" odd span, to get a good waterproof finish on any of the fabrics you need a good few coats of dope, followed by colour and most likely a two pot clear. Having done that, though, the fabrics do last and look good much longer than the films. If you wish to consider repair ease, then films are way ahead of the fabrics here. As for your floats. it is important to waterproof the insides, as much as the outsides. I don't think balsarite is waterproof when dry, better would be a polyurethane varnish which you can get from any good DIY store, cheap too. Koverite and dope on the outside, followed by a good two pot paint will keep the outside of the floats dry, and be tough enough to withstand the 'grass field' landings you will do. I know this 'cause my Brandenburg never got its feet wet, but spent over ten years sliding around on the float bottoms at the usual club fields! As for a replaceable wheel/float deal, of course it can be done, you have to do a bit of redesign so the legs can be unbolted and the float struts fitted, but as both sets use the same pickup points on the fuselage, that is no big deal. Having removeable floats makes transport much easier too.
Evan, WB #12.