You just need a perm address I believe. Then you can get plates.
(and much cheaper I might add.)I personally don't mind the influx of Californians or any other state for that matter. Of the people I have meet they all want the same thing... A better quality of living.
As per Nampa / Caldwell I have lived in both and do live in Caldwell. I know that the police departments in both cities have cracked down on all gang activity. Now don't get me wrong, I am not so naive to believe that all gangs are out of the cities. Also you will have gang activity in any city of any size.
As a basic rule of thumb the further west of Boise you go the cheaper the housing costs are. My wife and I just bought a brand new 1400 sq ft 3bed 2 bath 2 car garage home in east Caldwell for around $600 per month. If we would have waited 1-2 more weeks our payment would have been closer to $500/ month due to interest rates dropping.
Happy hunting and welcome to the treasure valley.