I am playing with a Moodle 2.0 instance and was wondering if there is any problem using the current nuvola set (that works so nicely with Moodle 1.9) with Moodle 2.0?
I don't think there is any problem. You can always copy over the icon images, and providing you put the folders in the right places, then they should work.
You might find this page useful:
My understanding (although I stand ready to be corrected on this) is that if you were to copy the icons across and replace those in one of the included core themes, those would be overwritten when updating your moodle.
If, however, they are in a custom theme, then the update would not overwrite them.
What I should have stressed but forgot to, was to say you only need to make the changes in your custom theme, and NOT in Moodle CORE or Moodle CORE themes as these wWILL be over-written during an upgrade.
Moodle will pick up the images you add to your custom theme's pix_plugins/mod/ folder much in the same way smart pix picked up mod icons in Moodle 1.9.
Adding these icon images to your custom theme is safe, providing of course you keep a copy of your theme, just in case future upgrades lose the theme folder!!!
What Richard is also saying is correct too.