I wouldn't do it. Sure you can keep them together until the cat grows and probably views the goldfish as dinner. Look up how they are caught and you will get the point. 800 gallons is not large enough for even one grown channel cat and goldfish. Channel cats get huge, are messy and need massive filtration. You will read on-line about people keeping them in aquariums and small ponds but in my opinion it is cruel. An above ground pond will probably get rather warm in Lodi, Ca where the summer temps get over 90 degrees. They like their water cooler.
Agree with Stephen Noble, they get large And will produce a lot of waste. I have a 5 year old channel cat in my 10,000 gallon pond it is about 2’ long and hoovers down food. I have it in there for what I hope is population control on the goldfish, koi and orfed that reproduce constantly.