Yes, it runs on all previously supported platforms, except the original WW1000 from Cyberguard.
is the WG 18.104.22.168 also compatible with the ww500e appliance?
It will run on a WW500, but not well if you don't have enough RAM. If you only have 1G RAM, it will run, but I wouldn't use it in a production environment.
Correct. While it basically works you always need to know that the WW500 is a Celeron powered device. I used to recommend customers to use WW500 with MWG7 for testing or as a cluster part for administration tasks. It will also work fine for very small environments, but should have 4 GB of memory if possible.
Just to be sure: Yes, it is supported. My recommendation is my personal opinion here.