The RTC gets to 4 feet plus, gulpers only reach 15 inches, max. Gulpers would be dinner, for sure. The photo you saw may not have been a full-grown RTC, which would probably work, but he's going to be full-grown fairly quickly, and the gulpers wouldn't have a chance.
As for side-viewing, I'm not sure how much it really matters with catfish. I don't mind not having a window, and I have Pacu in my pond, as well. If you look at my gallery, you'll see photos of my fish, and you can see the whole fish -- not just the top. When the fish is eight feet away from you, you're going to be able to see the whole thing -- if you were standing right over him, obviously, you'd only be able to see the top.
Oh, and as for the original question, I'd say a pond for an RTC and TSN, alone, would need to be about 1500 gallons, and you'd want as big of a footprint as you have room for. Keep in mind that you'll need to filter this with at least eight times volume, so 12,000 gallons per hour -- nothing to sneeze at when it comes to the electric bill, I assure you. I have 12,000 gph in my pond, and it's only 1,000 gallons, or somewhere around that. Also, you'll need a filter sized appropriately -- lots of room for biomedia, as well as mechanical filtration. We built ours into the pond's design in order to make it large enough.
I guess what I'm getting to is there's more than figuring out gallonage when you're planning a pond for big messy fish like these guys, so definitely don't forget to take everything into account.