RE: Max sail size for 76 Fish
The largest sail that I could put on my fish 76 was a 5.4 and on my Fish 90 was a 6.9 (but that was really pushing it). In both cases I threw away the thrusters and for the 6.0 used a 28 cm tru ames custom wave fin for my 83 kg and a 23 drake for my 63 kg wife. For the 5.4 I used a 24 and my wife a 19.
I hope this is of some reference for you. If you want my background info and formula for this send me an e-mail.