Hi Wayne,
My old F1, which I thnk would now be a CS6-2, but driven with a 5000. It runs on a 120g and seems to be keeping up fine. Just I would say though.
I would go to the 8in. model if it was me. On the Euroreef home page, the CS8-2 is listed as 24in. tall.
If you do use the CS6, then I would get it with the Sedra 5000 pump, instead of the 3500.
www.euro-reef.com/skimmers