RE: hybrid formula
8.5m will be no problem. 6.5m will also give the board forward motion, but only just!!!! Don't forget that at just under 16kg, the board is no light weight.
I guess you already have sails in the sizes you mention. If so, I suggest 8.5m is the one to use.
If you are thinking about which sail to buy? Then you should really think about a 9.5m freerace sail. The board is more than able to carry this size sail. I could even say that on the Hybrid, a 9.5m sail feels much smaller than it is.
At 75kg body weight, you will be fine.
However you mention 50kg also???? I am guessing that you are intending to use it for free riding? Maybe for smaller members of your family???? If so, then it's just a matter of logic.
In sub planing conditions anything from a tissue to a bedsheet will be fine. With 230 litres of volume, almost any size sail will get it going. The size and weight of the board, (don't forget it's 82cm width) conbined with its stability, makes it possible to control larger sails than you would perhaps use on other boards. So don't hesitate to rig up something a little larger.
I hope this will help you. It's a great board, and often under rated. It will last forever, and become more enjoyable each time you sail it.
P.s With the new phantom race boards now being introduced, I guess time is running out for the Hybrid. So if you want one, now would be the time to get one.