Your decision on James Hardi Plank is a good one. The installation should begin at the bottom and work up with an overhang over the bottom course. the gap should be very small if at all because the plank is about 1/4 inch. This means that the upper course is applied at a slight angle due to resting on the lower course and being nailed or screwed to the house walls. So, at one point the two pieces meet and have a tight seal.
If the gap is more than this, I would stop work and seek a professional person in this field and ask questions.