what happens if the floats are too short?
Just 5% ahead of the prop? I've seen up to 15% of the float ahead of the prop. Depends on CG and distance from CG to prop arc as a percentage of float length.
Now, since we're get into mounting considerations:
Bathtub test the floats to determine where the CG of the plane should fall in relation to the float length:
(Purchased floats should list where the aircraft's CG should be... you can go aft a bit... not forward.)
Wire the floats in position relative to each other, (as they will be when mounted) and set a weight on the floats (centered between... use a X of music wire to support the weight), roughly equal to the aircraft weight. Move the weight forward and aft to where it doesn't change the angle the floats ride in the water. CG of the plane goes from that point to very slightly aft of that point. (Full fuel tank CG shouldn't be forward of there.)
Its crude... but it works. If the plane's CG is too far forward (relative to the floats...) Yep, the plane's going to dig in every time, and you can't stop it. Too far aft, and you'll have problems getting "on plane," and thus take-offs will be difficult, but landings would be a bit easier, as it would be harder to accidentally nose-over. (up to a point... where things are just bad all around...)