27" or 28", an inch won't matter either way. Water rudders aren't necessary if you know how to work the throttle... shorts blasts to get the plane turned in the right direction then keep it moving, keep air moving over the rudder.
You might need to add a ventral fin under the tail for better yaw stability with the added cross-section of the floats.
Check out the Waterplane Forum
and the top sticky for plans and specs to set up a plane with floats.
Also, be sure to dunk your ESC in CorrosionX to make it waterproof or it'll fry if it gets wet.