Hi, but... I must import the pictures in Classic or Lr CC ?
That depends on what it is you want to accomplish. Do you want your full-sized images in the cloud, or do you want to just have shared collections of smart previews? If you want full-sized images in the cloud thing you need to import them using Lightroom CC. Do you also want copies of those images in Lightroom Classic CC? If you do then you need to configure Lightroom CC so that it also stores copies of those images on your local hard drive. Do you expect complete and full synchronization between Lightroom Classic CC and Lightroom CC at all times and under all circumstances? Then from my experience at least, prepare to be frustrated a great deal.
I have not migrated a catalog to Lightroom CC. I have imported a few images directly to Lightroom CC. I do most of my work using Lightroom Classic CC, and share collections of smart previews that I can work with in Lightroom CC. That is my preferred method.