Gas and low rpm is a great combination on an low compression forged bottom end, as the cylinder pressures are so low anyway, so its not a risk like it would be on say a zetec motor.

You can make the nitrous shutoff by using an rpm window switch, but i dont think you will want to when you try it, just leave it on all the way up the rev range.

A small hit will be ok in one go without needing to go progressive.

IF you go progressive you will need to get rid of that nasty american kit and get a wizards of nos one instead or you will be forver rebulding solenoids (6 mins life expectancy for a holly solenoid when pulsed vs an unlimited lifetime warranty on the wizards of nos one)