Hi Bill and welcome to the forums.
Christmas lights are completely dimmable as long as they aren't LEDs. I have done this a lot for different shows. An incandescent lamp is fully dimmable in a dimmer pack. I have likewise had regular old light bulbs that people use at home plugged into dimmers as well for shows.
As for your second question, running lights below their maxim output actually extends their life. So running par cans at say 80%, will last longer then running at 100%. Also, bumping lights shortens their life as well. You are more likely to blow a lamp on a bump then dimming it up over time. Sudden in-rush currents don't give the filament time to adjust and might break it.
However, running lights below maxim output increase lamp sing and dimmer buzz. It is the greatest at 50% since that is where the sine wave is chopped the most extreme. This doesn't hurt dimmers or lamps however, just annoying in sound sensitive environments. Likewise, if audio it catching dimmer buzz, it will be the loudest at 50%, and basically none at all at 0 or 100%.