Nevertheless, there are many paintbrushes for Krita. Is there a possibility to load only one certain group if I have many different groups? ZB only clouds.

At the moment, krita loads all brushes that are installed. You can select which brushes are shown using the tagging system (which is, I admit, buggy...).

What is the Tagging system?

Here: https://docs.krita.org/Tag_System