I'm sure you'll get lots of advice on this so I'll just toss this in for what it's worth....I sugget you balance the aircraft where there is no wind to eleminate the "pendulum effect".

You may want to add the weight to the floats so you aren't re-balancing the plane everytime you add or remove the floats. The most unobtrusive method is to add buckshot and epoxy to the nose of the float until it balances on the correct CG. You said "The plane hangs slightly tail down - I think", and it acted like it was tail heavy (elevator twitchy). It should balance level at the suggest CG per the manufacturer. Floats shouldn't change this. As for the up trim on the ailerons, if anything on a float plane I would crank in a little down trim on the ailerons (faux flaps). They aren't supposed to fly fast or do aerobatics, especially an 80" Cub.

You may also want to check where the step on the bottom of the floats ends up in relation to the CG. The step should be at the CG for a starting point.

As I said... just my opinion and you know what they say about opinions. Watch what a fire storm this thread starts. You have hit on at least three controversial topics.