It would probably be as easy to install Xfce in the MATE edition as it would be in MX-Linux. I had no issue with burning the Xfce version to an usb stick (also using MX's USB Maker). The only glitch I can guess at for your trouble is that little choice you make at the beginning of the process, where you choose "other than MX-Linux." It's an easy one to overlook. Reformat (FAT32) those USB sticks if they're still viable, but download, perhaps, from a different site. @ericbsd, our project leader, frequently visits this forum and if it turns out that the Canadian repo has an issue with the Xfce iso he'll discover it. Which leads to another question: I assume you did the sha check to make sure? You might also try torrenting, since it helps the project.
By the way, it's been a month or so since I started testing GhostBSD and I haven't even booted into MX-Linux since I started! It's that good!