can I use Dropbox and iCloud together?

Can you post a reference to this information to show us what Dropbox is saying?

DropBox and iCloud Drive are separate syncing services and run independently. I have used both and I've never had a problem.

A quick bit of digging on the Internet showed that you ought not have the two syncing the same folders, so that would potentially be messy. I don't use DropBox to its fullest. I find iCloud works well and is built into macOS and for that reason is the obvious one to use.