There is an Aperture plugin but I am not certain if it works with Lightroom CC. I haven't used it myself. They key is to have all of your images in a folder, or folder structure on a hard drive that Lightroom can access.
If you max out your cloud storage you can purchase additional storage space, or you can download images to a local drive and remove them from the cloud. If you take this approach those images will no longer be a part of your synced Lightroom ecosystem.
Classic is like a vacuum. Images in a Classic catalog MUST reside on a local hard drive. This is actually a nice feature when working with mobile. Any images you shoot with Lightroom Mobile will automatically download to a hard drive connected to Lightroom Classic. This happens after you turn on Lightroom Mobile Sync in the Lightroom catalog. You can set the location of the images in the Lightroom preferences.
- with the plan that includes both Classic and CC how are mobile files manages (ie can I push them to Classic or do I have to import them into the desktop app)
Photos shot on mobile devices automatically sync down into your LrClassic catalogue and hard drive. If you hit the 20gb limit, then you just unsync those mobile photos which removes the originals from the cloud and frees up space. If you still want them available on mobile devices you can then sync them again from LrClassic, which syncs smart previews, highly compressed files which don't count against the 20gb.
If you are an Aperture user, I would expect that you are likely to find the cloud-dependent LRCC underpowered, while LrClassic is much more comparable to how you have probably been using Aperture.