That mod will work fine. With the rudder extended to the bottom of the fuse, that allows you to drive the tailwheel off the bottom of the rudder, using a double horn on the rudder and connecting the tailwheel's double horn using springs. Be sure to divide the elevator far enough apart to allow for full travel on the rudder without hitting the elevator halves.

The alternative is to leave the tail as-is, and drive the tailwheel using a separate control rod off the rudder servo. It's the easier way to do it, but I like bashing things, and modifying the tail will at least make your kadet look a bit different. I have the kit (Kadet Sr.) and plan to modify the tail as you describe, but am going further and changing the outline as well, going to make a "poor man's" Helio Courier out of mine.

Here's what I have done on my plans. You can see that I not only lengthened the rudder, I also lengthened the V-stab and decreased its chord, too. I maintaine the same overall area of both. It's kind of hard to pick out, but if you look at the darker lines, you can see the new shape. The green lines are the actual mating lines, the V-stab will mount directly over the H stab, and the leading edges will meet, and then there will be that small transition on the V-stab added. I'll prbably drive my tailwheel off the rudder.

I plan on many other changes on my Senior, such as stretching the wing about 6 inches and removing most of the dihedral, addition of 3/4 span flaps, short ailerons, a tow release and "bomb bay", and provisions to mount a video system.