Annie, I bet you will be all right with the rudders David has designed for Siblim. I surely can see Paul’s point in making a balanced rudder for La Chica, but Siblim is so much lighter, and with fairly long tillers, so I think you will handle it easily (..btw, in Magazine 74, p.26, Karlis Kalnins describes a free-standing, balanced rudder with a little ‘deflector skeg’ to avoid catching ropes...).
Last summer I helped a friend with replacing the hinge roping on his Tiki 21. The original was probably well over twenty years old, and were still not really falling apart. We used 3mm Dyneema line. The new hinges turned out to be as good as any; nice and tight and with no steering friction.
If you ever find the tiller forces of Siblim to be on the high side, the straight trailing edge of the rudders would make it easy to fit a trimtab to one of them, and thus let you trim out any tiller forces. Even here, the use of rope hinges would simplify the job a lot.
Good luck!
Arne