Brad,
Not sure about question 1. 2d does not have a beam width to adjust. I think you are refering to DI or Down Imaging.
In that case I would say that wide will offer you the best images for objects or fish in the water column. If you are looking for more detail of structure on the bottom, narrow works better. And of course, med is in between.
As for SI, the optimum range with 800khz is 2 to 3 times your depth. This is because the beam in 800khz is smaller in the vertical than the 455khz and does not give you the full 180 deg coverage. In 455khz I would recommend 3 times your depth depending on your depth. You typically would not want a range of 150' in 10' of water if you are trying to spot fish. Maybe if you are looking for large structure yes.
Basically, you want to keep the range as low as possible without sacrificing too much area to the water column. But the larger the range, the smaller objects will appear on the screen.
Hope that helps a little,
Robert