The single servo mount should be fine. You fly a J-3 with the rudder anyway. The only tip I have for the floats is that you'll need to put a block in the back to hook it to, so a rough landing wont break it out. I dont really know how to switch between floats and wheel quickly, unless you had a completely different set of struts and everything for wheels and floats. That way you dont have to remove your axels.

Best bet for a beginner: Do it by the plans.

It is possible to put dual aileron servos out in the wings, but the installation of the mounts would require direct help from an experienced modeler.

The May 2002 Model Airplane News has an article which is related to putting floats on a J3. The information on float size selection and spacing will be helpful. it also shows a good scale appearing strut design which could be replicated, using less expensinve methods. That article has the best "How To" on getting floats aligned properly that I've seen in print.

The rear float struts need equal supporting structure in the fuselage to what is in place for the main landing gear. Again, refering to the photos in the above article will give you the correct location.

float flying is not for beginners... you need to be able to consistently do smooth take-offs and landings from grass or pavement before trying floats. Flipping the plane in the watter is never a minor problem... you can nose-over on land and often have no damage at all. In the water, if the plane flips, you aren't flying it again for quite a while. *(after you figure out how to get the plane out of the lake...)