Yes, he will. One of my friends has a 6.5' that eats everything, from stupid chickens, to the neighbors ducks (that go through three different barriers to prevent them from getting to the redtail's pond), to whole fish, even some turtles. He is around 120-150lbs. If it will fit in his mouth he will eat it.
Yes, fish are never full, anything they think is food will be tried - even if it was just fed not even two minutes ago. I've seen pictus catfish eat a small CAE right after having bloodworms where it was already nicely rounded with food.
Biggest thing is going to make sure it goes to a good home that will be able to house it long term. The one I my first post went from 12 inches to 4ft in about 8 months after being moved to the pond. Yes, the things the 6.5' one eats are all massive, the turtle it ate was 12" in diameter- gone in a gulp. Once he hits even 3ft he will eat the koI no matter the size, it will just make him grow faster with the surplus food. He will get massive, even full size koI won't be too much for him to swallow; koI are also too stupid to avoid a hunting catfish and don't have the instincts bass and other noncarp have to avoid being snacked on (I mean i've seen goldfish literally take food directly from a crayfish's mouth mandibles being right between the claws without even flinching away).
Catfish are not social with other species, doesn't matter how long they were together - if it fits in their mouth they will try (and most often) succeed in eating it.
3000 would be a short term solution as it would not be big enough for the cat long term.
I'd go with at least 10k gallons, not only for the mess it will make but to insure the pond is deep enough and has enough wiggle room for a full sized redtail cat. You can attempt to put a solid barrier in the bigger pond and make a section of the current pond the cat's permanent home, but you would have to really secure it.