Hello Niels,
To answer your first question, the use of the polar filter is not excluded. It does not necessarily bring big changes, but it may be enough for some nice shooting bright light because it will allow you to get the color stronger, especially with the blue, green and yellow . But we must also know that you risk losing quality sharpness.
Personally, I prefer to keep my focus and play with the colors in the shot. If you take pictures with beautiful lights start or end of the day, you will realize that it's possible to get wonderful colors using appropriate openings and moving differently depending on the natural light.
For your second question, the small rings (flares) can be obtained in several ways. Either naturally or artificially, by software post-processing.
I actually prefer the natural way. That's to say, that to succeed such effects background, you must be placed in backlight, with a little grazing light will come crashing drops of dew or rain.
It should also focus a large aperture (f/2.8 - f/3.2 - f/3.5 - f/4) to have well concentric circles. According to your direction and the number of drops of dew, you can get amazing backgrounds.
Here some exemples possible of backlights in drops with this natural technics :
http://1x.com/photo/115169/all:user:146644
http://1x.com/photo/101586/all:user:146644
http://1x.com/photo/99452/all:user:146644
http://1x.com/photo/128015/all:user:146644