I sail a '76 P26 with a Johnson 9.9 hp long shaft (20"). With the motor straight up - no tilt - the skeg will impede the rudder slightly if I am running at full throttle. If I adjust the tilt slightly (a peg or two), or do not run wide open, I have no problem. I definitely would not go with a smaller (less hp) motor.
Even with the long shaft the bottom of the motor comes out of the water in steep chop; this can be minimized by "going with the chop" (not quite sure how else to say that).
Boston Harbor, MA
1981 Hunter 33