The most versatile kicker brackets are retractable, i.e., they have a vertical travel of about 12". They should be set so that in the down position the prop is just below the bottom of the boat. That being the case, in the up position the prop should be out of the water while running on the main. Its not advisable to tilt a kicker on a bracket like that because you will eventually ruin the bracket and loosen the mounting bolts.
There are fixed mount brackets that allow tilting, but they would not be suitable on your boat considering the high freeboard.
Your 20" shaft should be just fine.
IBoats carries good brackets - pick the one that has the capacity for the weight and HP of your kicker.