"(1). How can I increase iso & keep noise to a minimum?"
I agree with the previous responses. If you are seeing too much noise in shots above ISO 400 then it is highly likely that you are underexposing. Increasing the exposure will reduce the visible noise. As also mentioned, check your shots occasionally to ensure you don't blow out any important highlights.
"(2). Should I be using auto iso in rapidly changing light situations?"
Some people really like auto ISO and others do not. The only way to find out if you like the feature is to try it out.