if I take a DNG photo using the Lightroom CC app on my iphone can I then send it to Lightroom CC Classic edit it and then export a JPG from Classic to Lightroom CC?
I would like to upload JPGs to Lightroom CC to have as many JPGs as possible in the Cloud so I don't waste so much space with all DNG Files.
If I can export a JPG from Lightroom Classic can I set a compression rate like say 50 percent so it's a smaller file size to save space in the cloud?
Yes and no. Classic doesn't upload originals, whether you convert to JPEG or now. What you could do is shoot the DNG file, let it download to Classic, remove it from sync (to remove the original from the cloud) and then add it back to sync from Classic, so it just uploads a smart preview (which is a 2560px compressed version, so would be smaller than a JPEG anyway).
how do you remove it from sync to the cloud? If I do so will it remove it from my computer where I assigned the files to go to?
Classic's a hoarder, so if it's in Classic, it'll stay in Classic. You'd remove it from the All Synced Photos collection, which would remove it from the cloud and all CC apps, but not from Classic.