Can I use images with a CC licence in my PowerPoint if the licences are different from the CC licence for the entire document?

Yes, you can. Each part of the presentation can be mentioned separately: copyright, Creative Commons or public domain. At the start or end of the presentation, the CC licence can be mentioned with the addition “full presentation unless otherwise stated” or a similar text. Make sure that each image with a different CC licence from the full presentation contains a link to the CC licence and that the correct source referencing is included if this is mandatory under the licence.

If you don't like the appearance of this in your presentation, you may add this information in the notes section under the slide or add a separate slide at the end of the presentation with the information listed for each slide.