You probably don't need the catfish. Goldfish are bottom feeders so they would do the same job.
Also, I recently read an account of a goldfish that tried to eat a small corydoras catfish and it got stuck in the fish's mouth and had to be cut out. I have no idea why the goldfish decided to try to eat it, but it didn't end well. It took several weeks of tube feeding for the fish's mouth to heal enough for it to eat on its own. I don't know what happened to the catfish. Maybe if the catfish were larger it wouldn't be a problem.
For a pond, it's generally recommended to have 50 gallons per goldfish. For an aquarium it seems to be 20 gallons. Not sure why the discrepancy there. It's also a good idea to have many times the recommended filtration, as much as 10 times, since goldfish are hefty and messy fish. Those common goldfish with good care will get to be 12" or more and the shubunkins 18". So that number will be right at the maximum or a bit over with the catfish.
Those 8 goldfish will soon turn into hundreds, so be aware of that.
Having no substrate is much easier to keep clean. Algae will soon cover the liner so it won't be visible anyway.